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/libsystemd-network/sd-dhcp-lease.c

@@ -26,6 +26,7 @@
 #include "stdio-util.h"
 #include "string-util.h"
 #include "strv.h"
+#include "tmpfile-util.h"
 #include "unaligned.h"
 
 int sd_dhcp_lease_get_address(sd_dhcp_lease *lease, struct in_addr *addr) {