Fix tar-in command hangs when running out of disk space (RHBZ#580246).

The problem was this sequence of events:

 (1) File transfer goes through OK.

 (2) pclose returns failure (because 'tar' subprocess failed)

 (3) We try to cancel the transfer by calling cancel_receive.

Step (3) fails because the transfer (as far as the library is
concerned) has succeeded, so causing a hang.

The more fundamental reason why we see steps (1) and (2) is that
'tar' does NOT fail immediately if there is a write error.  Instead
it continues reading and discarding the input until the end of the
input before giving "Error exit delayed from previous errors".
IMHO this is a bug with tar, since an ENOSPC write error should
be fatal for tar.