mime: add client reader

Add `mime` client reader. Encapsulates reading from mime parts, getting
their length, rewinding and unpausing.

- remove special mime handling from sendf.c and easy.c
- add general "unpause" method to client readers
- use new reader in http/imap/smtp
- make some mime functions static that are now only used internally

In addition:
- remove flag 'forbidchunk' as no longer needed

Closes #13039

lib/sendf.h

@@ -187,6 +187,7 @@ void Curl_cwriter_def_close(struct Curl_easy *data,
                             struct Curl_cwriter *writer);
 
 
+
 /* Client Reader Type, provides the implementation */
 struct Curl_crtype {
   const char *name;        /* writer name. */
@@ -200,6 +201,7 @@ struct Curl_crtype {
   CURLcode (*resume_from)(struct Curl_easy *data,
                           struct Curl_creader *reader, curl_off_t offset);
   CURLcode (*rewind)(struct Curl_easy *data, struct Curl_creader *reader);
+  CURLcode (*unpause)(struct Curl_easy *data, struct Curl_creader *reader);
   size_t creader_size;  /* sizeof() allocated struct Curl_creader */
 };
 
@@ -236,6 +238,8 @@ CURLcode Curl_creader_def_resume_from(struct Curl_easy *data,
                                       curl_off_t offset);
 CURLcode Curl_creader_def_rewind(struct Curl_easy *data,
                                  struct Curl_creader *reader);
+CURLcode Curl_creader_def_unpause(struct Curl_easy *data,
+                                  struct Curl_creader *reader);
 
 /**
  * Convenience method for calling `reader->do_read()` that
@@ -268,6 +272,14 @@ void Curl_creader_free(struct Curl_easy *data, struct Curl_creader *reader);
 CURLcode Curl_creader_add(struct Curl_easy *data,
                           struct Curl_creader *reader);
 
+/**
+ * Set the given reader, which needs to be of type CURL_CR_CLIENT,
+ * as the new first reader. Discard any installed readers and init
+ * the reader chain anew.
+ * The function takes ownership of `r`.
+ */
+CURLcode Curl_creader_set(struct Curl_easy *data, struct Curl_creader *r);
+
 /**
  * Read at most `blen` bytes at `buf` from the client.
  * @param date    the transfer to read client bytes for
@@ -328,6 +340,11 @@ curl_off_t Curl_creader_client_length(struct Curl_easy *data);
  */
 CURLcode Curl_creader_resume_from(struct Curl_easy *data, curl_off_t offset);
 
+/**
+ * Unpause all installed readers.
+ */
+CURLcode Curl_creader_unpause(struct Curl_easy *data);
+
 /**
  * Set the client reader to provide 0 bytes, immediate EOS.
  */