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.