core/service: when resetting PID also reset known flag

Re-watching pids on cgroup v1 (needed because of unreliability of cgroup
empty notifications in containers) is handled bellow at the end of
service_sigchld_event() and depends on value main_pid_known flag.

In CentOS Stream 8 container on cgroup v1 the stop action would get stuck
indefinitely on unit like this,

$ cat /run/systemd/system/foo.service
[Service]
ExecStart=/bin/bash -c 'trap "nohup sleep 1 & exit 0" TERM;  sleep infinity'
ExecStop=/bin/bash -c 'kill -s TERM $MAINPID'
TimeoutSec=0

However, upstream works "fine" because in upstream version of systemd we
actually never wait on processes killed in containers and proceed
immediately to sending SIGKILL hence re-watching of pids in the cgroup
is not necessary. But for the sake of correctness we should merge the
patch also upstream.

src/core/service.c

@@ -3766,6 +3766,7 @@ static void service_sigchld_event(Unit *u, pid_t pid, int code, int status) {
                         return;
 
                 s->main_pid = 0;
+                s->main_pid_known = false;
                 exec_status_exit(&s->main_exec_status, &s->exec_context, pid, code, status);
 
                 if (s->main_command) {