cgroups: refactor cgroup handling

cgroups: refactor cgroup handling

This replaces the constructor implementation of cgroup handling with a simpler,
thread-safe on-demand model of cgroup driver initialization.
Making the cgroup initialization code run in a constructor means that each time
the shared library gets mapped the cgroup parsing code gets run. That's
unnecessary overhead.
It also feels to me that this is only accidently thread-safe because
constructors are only run once. But should threads actually end up manipulating
or freeing memory that is file-global to cgfsng.c we'd be screwed. Now, I might
be wrong here but the cleaner implementation is to allocate a cgroup driver on
demand whenever we need it.
Take the chance and rework the cgroup_ops interface to make the functions it
wants to have implemented a lot cleaner.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>