process-util: introduce new FORK_FREEZE flag for safe_fork()

Often we want to fork off a process that just hangs until we kill it,
let's add a simple flag to create one of this type, and use it at
various places.

src/basic/process-util.h

@@ -192,6 +192,7 @@ typedef enum ForkFlags {
         FORK_NEW_USERNS         = 1 << 19, /* Run child in its own user namespace                                💣 DO NOT USE IN THREADED PROGRAMS! 💣 */
         FORK_NEW_NETNS          = 1 << 20, /* Run child in its own network namespace                             💣 DO NOT USE IN THREADED PROGRAMS! 💣 */
         FORK_NEW_PIDNS          = 1 << 21, /* Run child in its own PID namespace                                 💣 DO NOT USE IN THREADED PROGRAMS! 💣 */
+        FORK_FREEZE             = 1 << 22, /* Don't return in child, just call freeze() instead */
 } ForkFlags;
 
 int safe_fork_full(