xinitrc: Use 'exec' and comment logoff cleaning issue

.xinitrc

@@ -1,17 +1,28 @@
 #!/bin/sh
 
+## Run site init scripts. Usually not necessary.
+if [ -d /etc/X11/xinit/xinitrc.d ] ; then
+	for f in /etc/X11/xinit/xinitrc.d/?*.sh ; do
+		[ -x "$f" ] && . "$f"
+	done
+	unset f
+fi
+
 ## Source environment config.
 [ -f ~/.xprofile ] && . ~/.xprofile
 
 ## Launch session
-## Note: we do not 'exec' as we want to clean the session on exit.
 if [ $# -eq 1 ] && command -v $1 >/dev/null 2>&1; then
-	$1
+	exec $1
 else
-	awesome
+	exec awesome
 fi
 
-## TODO: This is not needed since all forked processes will be killed anyway
-## when 'exec' is not called. But why?
+## TODO: When the session is terminated, some forked processes must be closed
+## manually. This can automated by setting a shell 'trap' in .profile when
+## logged from a TTY. The trick does not work from an X login manager. This file
+## could be the place where to clean everything, but it will not be parsed by
+## some login managers. Beside, systemd sessions may sigkill all user apps on
+## logoff, preventing a clean termination.
 # command -v sessionclean >/dev/null 2>&1 && sessionclean