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/nspawn/nspawn-mount.c

@@ -6,7 +6,6 @@
 #include "alloc-util.h"
 #include "escape.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fs-util.h"
 #include "label.h"
 #include "mkdir.h"
@@ -20,6 +19,7 @@
 #include "stat-util.h"
 #include "string-util.h"
 #include "strv.h"
+#include "tmpfile-util.h"
 #include "user-util.h"
 #include "util.h"
 

src/nspawn/nspawn.c

@@ -93,6 +93,7 @@
 #include "string-util.h"
 #include "strv.h"
 #include "terminal-util.h"
+#include "tmpfile-util.h"
 #include "umask-util.h"
 #include "user-util.h"
 #include "util.h"