lxc-attach: unify code for attaching a pid to a cgroup

lxc-attach: unify code for attaching a pid to a cgroup

To attach a new pid to the cgroups for an existing container, we can use
the same method that we did when we started the container: iterate over
all the mounted cgroup hierarchies; find the cgroup that pid 1 is in for
each hierarchy; add 'lxc/<name>' to the end of it; then write the pid to
the 'tasks' file in that cgroup. (The only difference is that we do not
create the cgroup again.) Note that we follow exactly the same iteration
pattern to delete our cgroups when a container is shutdown.

There may be situations where additional cgroups hierarchies are mounted
after the container is started, or the cgroup for pid 1 gets reassigned.
But we currently don't handle any of these cases in the shutdown code or
anywhere else, so it doesn't make sense to try to handle these cases for
lxc-attach by itself. Aside from simplifying the code, this change makes
it easier to solve a different problem: ignoring hierarchies that are
not bound to any subsystems (like 'systemd').

Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>