lock-util: Add CLEANUP_UNPOSIX_UNLOCK()

Also migrate the logic in dynamic-user.c to use the new cleanup
macro.

src/shared/lock-util.c

@@ -149,3 +149,13 @@ int unposix_lock(int fd, int operation) {
 
         return r;
 }
+
+void unposix_unlockpp(int **fd) {
+        assert(fd);
+
+        if (!*fd || **fd < 0)
+                return;
+
+        (void) unposix_lock(**fd, LOCK_UN);
+        *fd = NULL;
+}

src/shared/lock-util.h

@@ -15,3 +15,8 @@ void release_lock_file(LockFile *f);
 
 /* Open File Description locks with the same interface as flock(). */
 int unposix_lock(int fd, int operation);
+
+void unposix_unlockpp(int **fd);
+
+#define CLEANUP_UNPOSIX_UNLOCK(fd)                                   \
+        _cleanup_(unposix_unlockpp) _unused_ int *CONCATENATE(_cleanup_unposix_unlock_, UNIQ) = &(fd)