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/test/test-stat-util.c

@@ -6,12 +6,12 @@
 
 #include "alloc-util.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "macro.h"
 #include "missing.h"
 #include "mountpoint-util.h"
-#include "stat-util.h"
 #include "path-util.h"
+#include "stat-util.h"
+#include "tmpfile-util.h"
 
 static void test_files_same(void) {
         _cleanup_close_ int fd = -1;