pcp

Fix a major botch in the Mac OS X kernel (darwin) PMDA.

Some macro cleanup during Windows porting has accidentally regressed
the darwin PMDA, causing it to fail to start thanks to some overly
strict checking on startup (pmda dispatch -> e_direct field) and a
newly introduced "hole" in the metric descriptor table thanks to the
removal of some no-longer-used metrics there.

Fix is to add back those metrics, just using not-supported types and
return codes on fetch, and be less strict during startup in case the
same class of error ever happens again (just warn instead of exiting
now, esp. since exiting is done in pmcd process context).

Issue found and fix tested by Ken, thanks!