process-util: add new FORK_NEW_MOUNTNS flag to safe_fork()

That way we can move one more code location to use safe_fork()

src/basic/process-util.h

@@ -168,6 +168,7 @@ typedef enum ForkFlags {
         FORK_REOPEN_LOG    = 1U << 4,
         FORK_LOG           = 1U << 5,
         FORK_WAIT          = 1U << 6,
+        FORK_NEW_MOUNTNS   = 1U << 7,
 } ForkFlags;
 
 int safe_fork_full(const char *name, const int except_fds[], size_t n_except_fds, ForkFlags flags, pid_t *ret_pid);