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/meson.build

@@ -139,6 +139,8 @@ shared_sources = files('''
         switch-root.h
         sysctl-util.c
         sysctl-util.h
+        tmpfile-util-label.c
+        tmpfile-util-label.h
         tomoyo-util.c
         tomoyo-util.h
         udev-util.c