more POSIX 2008 cleanup: strnlen(), rindex()

It has previously been established that POSIX 2008 is the baseline for
PostgreSQL code (modulo Windows). Looking through the POSIX 2008
changes, I found a few more things we can clean up with respect to that.

The first patch removes the configure checks for strnlen(), since that
is now required, and I see that all buildfarm members support. There
was some discussion in the commit messages that added these checks
(e.g., commit 8a241792f96) that suggests it was once required, but I did
not find any information about which platforms were affected then. But
that's 8 years ago now, so I think it might be obsolete.

The second patch replaces the single remaining use of rindex() with the
equivalent strrchr(). rindex() has been removed from POSIX.