libpcp: optimise DSO lookups for local context mode startup
libpcp: optimise DSO lookups for local context mode startup

Ensure we look first at the most-likely-valid DSO PMDA paths
to avoid unnecessary failing access(2) syscalls, when we're
establishing local contexts using the default DSO table.

Resolves Fedora BZ #1275293.

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