tmpfiles: also look in /lib/tmpfiles.d on split /usr systems

https://bugs.freedesktop.org/show_bug.cgi?id=38686

I don't think the usecase case in that bug makes much sense, but all the
other tools do honour /lib in the search path so we probably should do
that here, too.

src/tmpfiles/tmpfiles.c

@@ -107,6 +107,9 @@ static const char * const conf_file_dirs[] = {
         "/run/tmpfiles.d",
         "/usr/local/lib/tmpfiles.d",
         "/usr/lib/tmpfiles.d",
+#ifdef HAVE_SPLIT_USR
+        "/lib/tmpfiles.d",
+#endif
         NULL
 };