pcp

MMV client API improvements.

This patch:

• adds an API routine for setting the value directly, which is
  needed for instantaneous values (not just incrementing then);
• moves the mmv_stats inline wrapper routines into the library,
  simplifying the main API header a whole lot;
• directly use the pmapi.h macros where they match up with some
  mmv_stats.h macros, ensuring values match & meaning is clear;
• update the Perl client module to remove the dependence on the
  PMDA module;
• add a decent Perl example showing how to instrument with this
  module;
• add an "elapsed time" metric type which correctly handles the
  in-progress timer situation.

Ondisk format and client APIs are now feature complete for us.