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/request.h

@@ -139,9 +139,6 @@ struct SingleRequest {
   BIT(upload_chunky); /* set TRUE if we are doing chunked transfer-encoding
                          on upload */
   BIT(getheader);    /* TRUE if header parsing is wanted */
-  BIT(forbidchunk);  /* used only to explicitly forbid chunk-upload for
-                        specific upload buffers. See readmoredata() in http.c
-                        for details. */
   BIT(no_body);      /* the response has no body */
   BIT(authneg);      /* TRUE when the auth phase has started, which means
                         that we are creating a request with an auth header,