pcp

In-progress commit of cgroup metrics for Linux kernel PMDA.

This makes the following changes to pmdalinux, as we move toward
properly supporting cgroups:

• keep track of mount options provided to filesystems (needed to tell which cgroup subsystems in use for a cgroup mount point).
• add a filesys.readonly metric now that this is trivial to do.
• refactor the proc metric code so that we can use subsets of all PIDs instead of all PIDs (to be used later by cgroup task metrics)
• added initial metrics for cgroups - subsystems and mounts.
• switch indom definitions from cpp macros to enum like clusters, add in comments explaining what each one is.