DONE: consider callback-aborted transfers premature

This bug report properly identified that when doing SMTP and aborting
the transfer with a callback, it must be considered aborted prematurely
by the code to avoid QUIT etc to be attempted as that would cause a
hang.

The new test case 1507 verifies this behavior.

Reported by: Patricia Muscalu
Bug: http://curl.haxx.se/bug/view.cgi?id=1184