pmrep: unify archive recording

pmrep: unify archive recording

Even before the first release pmrep already has two archive recording
modes due to "historical reasons" :( I'd rather fix this before the
release. (I used pmcollectl's pmgui based approach as an example
initially but it doesn't cope with the from-archive-to-archive mode so
the pmi based approach was recently added.)

The pmgui method has the tiny "off-sync" issue mentioned earlier [1],
the pmi method doesn't suffer this as it simply skips those records,
apparently to no harm [2].

By switching to pmi in all cases, we can avoid duplication and avoid
this minor issue.

But perhaps most importantly, pmi will output archives not archive
folios (and doesn't create those config/log files) so it should be
easier for users as well.

1) http://oss.sgi.com/pipermail/pcp/2015-November/008741.html
2) http://oss.sgi.com/pipermail/pcp/2015-December/008892.html