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/shared/tmpfile-util-label.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <stdio.h>
+
+/* These functions are split out of tmpfile-util.h (and not for example just flags to the functions they wrap) in order
+ * to optimize linking: This way, -lselinux is needed only for the callers of these functions that need selinux, but
+ * not for all */
+
+int fopen_temporary_label(const char *target, const char *path, FILE **f, char **temp_path);