formdata: ack read callback abort

When doing a multipart formpost with a read callback, and that callback
returns CURL_READFUNC_ABORT, that return code must be properly
propagated back and handled accordingly. Previously it would be handled
as a zero byte read which would cause a hang!

Added test case 587 to verify. It uses the lib554.c source code with a
small ifdef.

Reported by: Anton Bychkov
Bug: http://curl.haxx.se/mail/lib-2011-10/0097.html