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

tests/data/test587

@@ -0,0 +1,51 @@
+<testcase>
+#
+# Server-side
+<reply>
+<data>
+</data>
+</reply>
+
+# Client-side
+<client>
+<server>
+http
+</server>
+# tool is what to use instead of 'curl'
+<tool>
+lib587
+</tool>
+
+ <name>
+HTTP multi-part formpost with aborted read callback
+ </name>
+ <command>
+http://%HOSTIP:%HTTPPORT/587
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strippart>
+s/^------------------------------[a-z0-9]*/------------------------------/
+s/boundary=----------------------------[a-z0-9]*/boundary=----------------------------/
+</strippart>
+<protocol>
+POST /587 HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+Accept: */*
+Content-Length: 732
+Expect: 100-continue
+Content-Type: multipart/form-data; boundary=----------------------------
+
+------------------------------
+Content-Disposition: form-data; name="sendfile"; filename="postit2.c"
+
+</protocol>
+# CURLE_ABORTED_BY_CALLBACK (42)
+<errorcode>
+42
+</errorcode>
+</verify>
+</testcase>