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.c

@@ -1763,6 +1763,9 @@ int safe_fork_full(
                 }
         }
 
+        if (FLAGS_SET(flags, FORK_FREEZE))
+                freeze();
+
         if (ret_pid)
                 *ret_pid = getpid_cached();