Change startup mode of zed under systemd (revisited)

This is a follow-up to #2252 from @Lalufu for changing the zed startup mode under systemd from forking to simple/non-forking.  It removes the program identity prefix from messages written to stderr since systemd already prepends this output with the program name.  It also fixes a race-condition with systemd when creating the PID file in forking mode.

Replaces #2252.