service: watch main pid even in final states

In some circumstances, for example when start-up times out we
immediately jump into the final state, at which point we still should
try to watch the main pid so that the SIGCHLD allows us to quickly
move into dead state.

src/shared/util.c

@@ -6088,3 +6088,13 @@ int namespace_enter(int pidns_fd, int mntns_fd, int root_fd) {
 
         return 0;
 }
+
+bool pid_valid(pid_t pid) {
+        if (pid <= 0)
+                return false;
+
+        if (kill(pid, 0) >= 0)
+                return true;
+
+        return errno != ESRCH;
+}

src/shared/util.h

@@ -804,3 +804,5 @@ int container_get_leader(const char *machine, pid_t *pid);
 
 int namespace_open(pid_t pid, int *pidns_fd, int *mntns_fd, int *root_fd);
 int namespace_enter(int pidns_fd, int mntns_fd, int root_fd);
+
+bool pid_valid(pid_t pid);