units: mount /tmp as tmpfs

The default setups should be a stateless as possible. /tmp as tmpfs is
the intended default for general purpose systems.

Small temporary files should not be stored on disk; lager files, or
files which should potentially survive a reboot, belong into /var/tmp.

Also catch up with some good old UNIX history.

More details are here:
  https://fedoraproject.org/wiki/Features/tmp-on-tmpfs

Makefile.am

@@ -289,6 +289,7 @@ dist_systemunit_DATA = \
 	units/sys-kernel-config.mount \
 	units/sys-kernel-debug.mount \
 	units/sys-fs-fuse-connections.mount \
+	units/tmp.mount \
 	units/remount-rootfs.service \
 	units/printer.target \
 	units/sound.target \
@@ -2275,10 +2276,12 @@ systemd-install-data-hook:
 	( cd $(DESTDIR)$(systemunitdir)/local-fs.target.wants && \
 		rm -f systemd-remount-api-vfs.service \
 			fsck-root.service \
-			remount-rootfs.service && \
+			remount-rootfs.service \
+			tmp.mount && \
 		$(LN_S) ../systemd-remount-api-vfs.service systemd-remount-api-vfs.service && \
 		$(LN_S) ../fsck-root.service fsck-root.service && \
-		$(LN_S) ../remount-rootfs.service remount-rootfs.service )
+		$(LN_S) ../remount-rootfs.service remount-rootfs.service && \
+		$(LN_S) ../tmp.mount tmp.mount )
 	( cd $(DESTDIR)$(userunitdir) && \
 		rm -f shutdown.target sockets.target bluetooth.target printer.target sound.target && \
 		$(LN_S) $(systemunitdir)/shutdown.target shutdown.target && \

units/tmp.mount

@@ -0,0 +1,16 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU General Public License as published by
+#  the Free Software Foundation; either version 2 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Temporary Directory
+Before=local-fs.target
+
+[Mount]
+What=tmpfs
+Where=/tmp
+Type=tmpfs
+Options=mode=1777