pcp

Format extensions for the Memory Mapped Value PMDA and library.

This change introduces several changes to the ondisk format, and
we are much closer to having a releasable next version:

• support for string values
• support for metric help text (short/long)
• removal of mmv.conf and support for client-driven cluster IDs (client prefered cluster ID stored in header)
• additional flags field in the initial header, allowing:
• option for not exporting values if intrumented PID not running (creators pid stored in header)
• fixed item IDs stored ondisk, for persistent PMID numbering.