let lxc-monitor command ask a lxc-monitord instance to quit

let lxc-monitor command ask a lxc-monitord instance to quit

Once lxc-monitord receives a quit request from lxc-monitor, it will then
return from the mainloop every time an event occurs on any of its fds and
check if it has any clients left. When there are no more it exits. This
allows lxc-monitord to quit immediately instead of waiting the normal 30
seconds for more clients, potentially freeing up lxcpath for unmounting.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>