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>

src/Makefile.am

@@ -59,6 +59,7 @@ finit_SOURCES      = api.c	cgroup.c	cgroup.h	\
 		     sig.c	sig.h				\
 		     sm.c	sm.h				\
 		     svc.c	svc.h				\
+		     tmpfiles.c	tmpfiles.h			\
 		     tty.c	tty.h				\
 		     util.c	util.h				\
 		     utmp-api.c	utmp-api.h