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.c

@@ -139,7 +139,6 @@ void Curl_req_reset(struct SingleRequest *req, struct Curl_easy *data)
   req->ignore_cl = FALSE;
   req->upload_chunky = FALSE;
   req->getheader = FALSE;
-  req->forbidchunk = FALSE;
   req->no_body = data->set.opt_no_body;
   req->authneg = FALSE;
 }