Add support for systemd system state notifications

When the systemd dev package (e.g. libsystemd-dev) is installed, we can use
sd_notify() to inform systemd of Synchronet server state/status. The state
values (READY, STOPPING, RELOADING) are "well known" while the status
value is a free-form text string (the last lputs output).

Eliminated the free-form status() callback from *startup_t.
Eliminated the now-redundant started() callback from *startup_t.

Eliminated the use of sbbs_status.c/h as nothing is using that interface
and while I do plan on using a remote control/monitor interface, I'm
considering use of a standard pub/sub lib and protocol.

This commit is going to break the Windows sbbsctrl build for sure.
Probably break the sbbsNTsvcs build.