mime: implement encoders.

curl_mime_encoder() is operational and documented.
curl tool -F option is extended with ";encoder=".
curl tool --libcurl option generates calls to curl_mime_encoder().
New encoder tests 648 & 649.
Test 1404 extended with an encoder specification.

docs/cmdline-opts/form.d

@@ -101,6 +101,20 @@ text file:
 .br
       -F '=)' -F '=@textfile.txt' ...  smtp://example.com
 
+Data can be encoded for transfer using encoder=. Available encodings are
+\fIbinary\fP and \fI8bit\fP that do nothing else than adding the corresponding
+Content-Transfer-Encoding header, \fI7bit\fP that only rejects 8-bit characters
+with a transfer error, \fIquoted-printable\fP and \fIbase64\fP that encodes
+data according to the corresponding schemes, limiting lines length to
+76 characters.
+
+Example: send multipart mail with a quoted-printable text message and a
+base64 attached file:
+
+ curl -F '=text message;encoder=quoted-printable' \\
+.br
+      -F '=@localfile;encoder=base64' ... smtp://example.com
+
 See further examples and details in the MANUAL.
 
 This option can be used multiple times.

docs/libcurl/Makefile.inc

@@ -21,4 +21,4 @@ man_MANS = curl_easy_cleanup.3 curl_easy_getinfo.3 curl_easy_init.3      \
   curl_mime_init.3 curl_mime_free.3 curl_mime_addpart.3 curl_mime_name.3 \
   curl_mime_data.3 curl_mime_data_cb.3 curl_mime_filedata.3              \
   curl_mime_filename.3 curl_mime_subparts.3                              \
-  curl_mime_type.3 curl_mime_headers.3
+  curl_mime_type.3 curl_mime_headers.3 curl_mime_encoder.3

docs/libcurl/curl_mime_addpart.3

@@ -62,4 +62,5 @@ A mime part structure handle, or NULL upon failure.
 .BR curl_mime_filename "(3),"
 .BR curl_mime_subparts "(3),"
 .BR curl_mime_type "(3),"
-.BR curl_mime_headers "(3)"
+.BR curl_mime_headers "(3),"
+.BR curl_mime_encoder "(3)"

docs/libcurl/curl_mime_encoder.3

@@ -0,0 +1,97 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.TH curl_mime_encoder 3 "22 August 2017" "libcurl 7.56.0" "libcurl Manual"
+.SH NAME
+curl_mime_encoder - set a mime part's encoder and content transfer encoding
+.SH SYNOPSIS
+.B #include <curl/curl.h>
+.sp
+.BI "CURLcode curl_mime_encoder(curl_mimepart * " part ,
+.BI "const char * " encoding ");"
+.ad
+.SH DESCRIPTION
+curl_mime_encoder() requests a mime part's content to be encoded before being
+transmitted.
+
+\fIpart\fP is the part's handle to assign an encoder.
+\fIencoding\fP is a pointer to a zero-terminated encoding scheme. It may be
+set to NULL to disable an encoder previously attached to the part. The encoding
+scheme storage may safely be reused after this function returns.
+
+Setting a part's encoder twice is valid: only the value set by the last call is
+retained.
+
+Upon multipart rendering, the part's content is encoded according to the
+pertaining scheme and a corresponding \fIContent-Transfer-Encoding"\fP header
+is added to the part.
+
+Supported encoding schemes are:
+.br
+"\fIbinary\fP": the data is left unchanged, the header is added.
+.br
+"\fI8bit\fP": header added, no data change.
+.br
+"\fI7bit\fP": the data is unchanged, but is each byte is checked
+to be a 7-bit value; if not, a read error occurs.
+.br
+"\fIbase64\fP": Data is converted to base64 encoding, then split in
+CRLF-terminated lines of at most 76 characters.
+.br
+"\fIquoted-printable\fP": data is encoded in quoted printable lines of
+at most 76 characters. Since the resulting size of the final data cannot be
+determined prior to reading the original data, it is left as unknown, causing
+chunked transfer in HTTP. For the same reason, this encoder may not be used
+with IMAP. This encoder targets text data that is mostly ASCII and should
+not be used with other types of data.
+
+If the original data is already encoded in such a scheme, a custom
+\fIContent-Transfer-Encoding\fP header should be added with
+\FIcurl_mime_headers\fP() instead of setting a part encoder.
+
+Encoding should not be applied to multiparts, thus the use of this
+function on a part with content set with \fIcurl_mime_subparts\fP() is
+strongly discouraged.
+.SH AVAILABILITY
+As long as at least one of HTTP, SMTP or IMAP is enabled. Added in 7.56.0.
+.SH RETURN VALUE
+CURLE_OK or a CURL error code upon failure.
+.SH EXAMPLE
+.nf
+ curl_mime *mime;
+ curl_mimepart *part;
+
+ /* create a mime handle */
+ mime = curl_mime_init(easy);
+
+ /* add a part */
+ part = curl_mime_addpart(mime);
+
+ /* send a file */
+ curl_mime_filedata(part, "image.png");
+
+ /* encode file data in base64 for transfer */
+ curl_mime_encoder(part, "base64");
+.fi
+.SH "SEE ALSO"
+.BR curl_mime_addpart "(3),"
+.BR curl_mime_headers "(3),"
+.BR curl_mime_subparts "(3)"