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/basic/copy.c

@@ -19,7 +19,6 @@
 #include "copy.h"
 #include "dirent-util.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fs-util.h"
 #include "io-util.h"
 #include "macro.h"
@@ -28,6 +27,7 @@
 #include "string-util.h"
 #include "strv.h"
 #include "time-util.h"
+#include "tmpfile-util.h"
 #include "umask-util.h"
 #include "user-util.h"
 #include "xattr-util.h"