uuidd: implement --socket-activation option

uuidd: implement --socket-activation option

Implement --socket-activation option, which is supposed to be used
with systemd (or equivalent init system) socket activation mechanism.
With this option, the UUID daemon does not create the socket on its own,
but instead expect the connection fd to be provided by the calling
process.

This option is only available if util-linux is configured with
--enable-socket-activation option. With this configure
option, the uuidd is compiled with sd-daemon.c.

Signed-off-by: Petr Uzel <petr.uzel@suse.cz>