ubusd: fix systemd socket activation support
62cdfc3 added systemd units including a ubus.socket unit, but didn't
actually add socket activation support to ubusd. This would cause the first
connection that activated ubusd to hang (as ubusd ignored it), and stopping
ubusd would break it completely (as ubusd removed the socket file).
The ENABLE_SYSTEMD default is changed to OFF as the socket activation uses
libsystemd, so setting ENABLE_SYSTEMD to ON will now require libsystemd.
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>