tmpfiles: Move /tmp and /var/tmp to a separate tmpfiles.d file to ease overrides via /etc

Many people prefer to avoid clearing /tmp and /var/tmp, and
distributions often have explicit settings for how often to clear them
if at all.  Overriding those with systemd currently requires overriding
all of /usr/lib/tmpfiles.d/systemd.conf via
/etc/tmpfiles.d/systemd.conf, copying across all the other entries, and
updating that override when systemd.conf changes.

Move the /tmp and /var/tmp entries from systemd.conf to a separate
tmp.conf, making them easier to override without affecting the rest of
systemd.conf.

Makefile.am

@@ -301,6 +301,7 @@ dist_bashcompletion_DATA = \
 
 dist_tmpfiles_DATA = \
 	tmpfiles.d/systemd.conf \
+	tmpfiles.d/tmp.conf \
 	tmpfiles.d/x11.conf
 
 if HAVE_SYSV_COMPAT

tmpfiles.d/systemd.conf

@@ -13,9 +13,6 @@ F /run/utmp 0664 root utmp -
 f /var/log/wtmp 0664 root utmp -
 f /var/log/btmp 0600 root utmp -
 
-d /tmp 1777 root root 10d
-d /var/tmp 1777 root root 30d
-
 d /var/cache/man - - - 30d
 
 r /forcefsck

tmpfiles.d/tmp.conf

@@ -0,0 +1,12 @@
+#  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.
+
+# See tmpfiles.d(5) for details
+
+# Clear tmp directories separately, to make them easier to override
+d /tmp 1777 root root 10d
+d /var/tmp 1777 root root 30d