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/systemctl/systemctl.c

@@ -37,7 +37,6 @@
 #include "escape.h"
 #include "exit-status.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "format-util.h"
 #include "fs-util.h"
 #include "glob-util.h"
@@ -73,6 +72,7 @@
 #include "string-table.h"
 #include "strv.h"
 #include "terminal-util.h"
+#include "tmpfile-util.h"
 #include "unit-def.h"
 #include "unit-name.h"
 #include "user-util.h"