tree-wide: add a few missing _sentinel_ decorators

src/basic/process-util.h

@@ -159,7 +159,7 @@ static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
         return safe_fork_full(name, NULL, 0, flags, ret_pid);
 }
 
-int fork_agent(const char *name, const int except[], size_t n_except, pid_t *pid, const char *path, ...);
+int fork_agent(const char *name, const int except[], size_t n_except, pid_t *pid, const char *path, ...) _sentinel_;
 
 int set_oom_score_adjust(int value);