implementations of dup3 and fallocate that are good enough to fool LTP

updated fallocate check to new configure, added dup3 check as suggested
by Jan-Simon Möller.

Riku: updated to apply to current git.

Signed-off-by: Ulrich Hecht <uli@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
