pcp-numastat: use pmFetchArchive when replaying an archive
pcp-numastat: use pmFetchArchive when replaying an archive

In archive mode, pcp-numastat fails if the required metrics are
not in the first record of the archive and an offset (-O) or starting
position (-S) isn't specified. So use pmFetchArchive in this case.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp