util-lib: split out all temporary file related calls into tmpfiles-util.c

This splits out a bunch of functions from fileio.c that have to do with
temporary files. Simply to make the header files a bit shorter, and to
group things more nicely.

No code changes, just some rearranging of source files.

src/timedate/timedated.c

@@ -15,6 +15,7 @@
 #include "clock-util.h"
 #include "def.h"
 #include "fileio-label.h"
+#include "fileio.h"
 #include "fs-util.h"
 #include "hashmap.h"
 #include "list.h"