doc updates re: restart/reload/systemd
Try to deal with systemd + restart action a little better

(Also, use $MAINPID as a backup check for control procs running
under systemd, since NotifyAccess=all doesn't currently set
$NOTIFY_SOCKET for them).
expose dmn_sd_notify, use for STOPPING=1

(which should be supported for systemd 217+, and ignored
in earlier versions).