lxc-usernsexec: add a manpage

and fix the help output in the program

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Deprecate lxc-checkpoint, lxc-kill and lxc-restart

Checkpoint/restart isn't currently supported, so let's not carry those
binaries around until we have proper CRIU support in the API.

lxc-kill is redundant with lxc-stop -k and has been known to confuse user.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>