Simplify strchrnul() compat code

Simplify strchrnul() compat code

strchrnul() was introduced in glibc in April 1999 and included in
glibc-2.1. Checking for that version means the majority of all git
users would get to use the optimized version in glibc. Of the
remaining few some might get to use a slightly slower version
than necessary but probably not slower than what we have today.

Unfortunately, __GLIBC_PREREQ() macro was not available in glibc 2.1.1
which was short lived but already supported strchrnul().  Odd minority
users of that library needs to live with our compatibility inline version.

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