Replace a few strncmp() calls with strlcpy().

Replace a few strncmp() calls with strlcpy().

strncmp() is a specialized API unsuited for routine copying into
fixed-size buffers.  On a system where the length of a single filename
can exceed MAXPGPATH, the pg_archivecleanup change prevents a simple
crash in the subsequent strlen().  Few filesystems support names that
long, and calling pg_archivecleanup with untrusted input is still not a
credible use case.  Therefore, no back-patch.

David Rowley