Add strchrnul()

Add strchrnul()

As suggested by Pierre Habouzit, add strchrnul().  It's a useful GNU
extension and can simplify string parser code.  There are several
places in git that can be converted to strchrnul(); as a trivial
example, this patch introduces its usage to builtin-fetch--tool.c.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>