server: Use "normal" socket activation, not inetd

Systemd, when doing socket activation, pass file descriptors in a
non-stable order when there is multiples ones.

But we only use one, so we don't need to identify it, and the file
descriptors always start at 3.
So use 3 for the systemd service.

Source : sd_listen_fds (systemd man pages)

We also need to unset variables systemd pass to socket activated
process, since we don't need them and sub-process (footclient and
theirs forks) could be confused by those.

Closes #1107

foot-server@.service.in

@@ -1,8 +1,8 @@
 [Service]
-ExecStart=@bindir@/foot --server=0
+ExecStart=@bindir@/foot --server=3
 Environment=WAYLAND_DISPLAY=%i
+UnsetEnvironment=LISTEN_PID LISTEN_FDS LISTEN_FDNAMES
 NonBlocking=true
-StandardInput=socket
 
 [Unit]
 Requires=%N.socket