Use ff_neterrno instead of errno in tcp.c.

Use ff_neterrno instead of errno in tcp.c.
If ff_neterrno() is checked instead of errno, that's probably what should be
used in the return value.
As a sideeffect, this gives better compatibility with Windows (CE), where
network errors aren't visible in errno.)
patch by Martin Storsjö, martin martin st

Originally committed as revision 19433 to svn://svn.ffmpeg.org/ffmpeg/trunk