Make fopen_temporary and fopen_temporary_label unlocked

This is partially a refactoring, but also makes many more places use
unlocked operations implicitly, i.e. all users of fopen_temporary().
AFAICT, the uses are always for short-lived files which are not shared
externally, and are just used within the same context. Locking is not
necessary.

src/basic/mountpoint-util.c

@@ -2,7 +2,6 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#include <stdio_ext.h>
 #include <sys/mount.h>
 
 #include "alloc-util.h"