Improve behaviour for unprivileged users

Improve behaviour for unprivileged users

This mostly changes two things:
 - Only log to the container's logfile on start/stop/restart/execute
 - Call may_control() every time we use the API and return
   "Insufficient privileges" on failure.

NOTE: I didn't test every single one of those but I'm fairly confident
      in my copy/paste abilities and I confirmed they all build fine at least.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>