commands: make state server interface flexible

commands: make state server interface flexible

This adds a little more flexibility to the state server. The idea is to have a
command socket function "lxc_cmd_add_state_client()" whose only task is to add
a new state client to the container's in-memory handler. This function returns
either the state of the container if it is already in the requested state or it
will return the newly registered client's fd in one of its arguments to the
caller. We then provide a separate helper function "lxc_cmd_sock_rcv_state()"
which can be passed the returned client fd and listens on the fd for the
requested state.
This is useful when we want to first register a client, then send a signal to
the container and wait for a state. This ensure that the client fd is
registered before the signal can have any effect and can e.g. be used to catch
something like the "STOPPING" state that is very ephemeral.

Additionally we provide a convenience function "lxc_cmd_sock_get_state()" which
combines both tasks and is used in e.g. "lxc_wait()".

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