mime: latch last read callback status.

In case a read callback returns a status (pause, abort, eof,
error) instead of a byte count, drain the bytes read so far but
remember this status for further processing.
Takes care of not losing data when pausing, and properly resume a
paused mime structure when requested.
New tests 670-673 check unpausing cases, with easy or multi
interface and mime or form api.

Fixes #4813
Reported-by: MrdUkk on github

tests/data/test644

@@ -50,6 +50,7 @@ Content-Type: multipart/form-data; boundary=----------------------------
 ------------------------------
 Content-Disposition: form-data; name="sendfile"; filename="postit2.c"
 
+
 </protocol>
 # CURLE_ABORTED_BY_CALLBACK (42)
 <errorcode>