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-async.c

@@ -3,8 +3,8 @@
 #include <unistd.h>
 
 #include "async.h"
-#include "fileio.h"
 #include "macro.h"
+#include "tmpfile-util.h"
 #include "util.h"
 
 static bool test_async = false;