Add limited tmpfiles.d(5) support

This change adds very basic tmpfiles.d/ support to Finit.  Much of the
basic types are supported, but not all, so for now, please check the
code for details on what is working.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

tmpfiles.d/etc.conf

@@ -0,0 +1,7 @@
+L  /etc/os-release      - - - - ../usr/lib/os-release
+L+ /etc/mtab		- - - - ../proc/self/mounts
+C! /etc/locale.conf	- - - -
+C! /etc/nsswitch.conf	- - - -
+C! /etc/vconsole.conf	- - - -
+C! /etc/pam.d		- - - -
+C! /etc/issue		- - - -