pcp

Event records support

This batch of changes incorporates Nathan's review and some additional
work to generalize the support for event records with

• additional event flag definitions
• pmEventFlagsStr() added to libpcp
• assorted pmdaEvent* routines extracted from the sample PMDA, made more generic and added to libpcp_pmda
• man pages for the above changes
• assorted consequential changes to the sample PMDA, pminfo and pmdumplog