systemd notification support

tgtd may take some time to initialize before accepting management
commands. Since management commands are used to bring the targets up
during service startup, we have to make sure that tgtd is responsive
before proceeding. To this end, we add a call to sd_notify(3) right
before entering the event loop to signal systemd (if applicable) that
the main process is ready.

Systemd support with the relevant includes and linker flags is optional,
controlled by the SD_NOTIFY make flag.

Signed-off-by: Apollon Oikonomopoulos <apoikos@debian.org>