test-pipe: make a bit more robust.

test-pipe: make a bit more robust.

* tests/test-pipe.c (myerr): Allow error messages regardless of
what we do to stderr.
(test_pipe): Rearrange to avoid deadlock.
(child_main): Try a larger read, to ensure we avoided deadlock.
* lib/pipe.c (create_pipe) [_WIN32]: Fix comment.
* lib/pipe.h (create_pipe_bidi): Document potential for deadlock
if misused.

Signed-off-by: Eric Blake <ebb9@byu.net>