- refactor cgroup into two backends, the classic cgfs driver and the new
cgmanager. Instead of lxc_handler knowing about the internals of each,
have it just store an opaque pointer to a struct that is private to
each backend.
- rename a couple of cgroup functions for consistency: those that are
considered an API (ie. exported by lxc.h) begin with lxc_ and those that
are not are just cgroup_*
- made as many backend routines static as possible, only cg*_ops_init is
exported
- made a nrtasks op which is needed by the utmp code for monitoring
container shutdown, currently only implemented for the cgfs backend