Make use of xlog_internal.h's macros in WAL-related utilities.

Make use of xlog_internal.h's macros in WAL-related utilities.

Commit 179cdd09 added macros to check if a filename is a WAL segment
or other such file. However there were still some instances of the
strlen + strspn combination to check for that in WAL-related utilities
like pg_archivecleanup. Those checks can be replaced with the macros.

This patch makes use of the macros in those utilities and
which would make the code a bit easier to read.

Back-patch to 9.5.

Michael Paquier