Enabling the "systemd" feature will enable the socket activation
feature used by systemd. This way we can (re-)start the GPSD based
on the socket activation.
Henning got really confused that the "a.b.c:1234" he passed to
gps2udp showed up as "a.b.c 1234" in the kernel cmdline. Use
strdup to take a copy of it.
Fixes: SYS#424
Holger Hans Peter Freyther
Henning Heinold