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.

(cherry picked from commit ff32060f2ed37b68dc26256b05e2e69013b0ecfe)
(cherry picked from commit ae83e97a51519ca33e70d7ba142cb3ed24212825)
(cherry picked from commit 03c270fa6bf0488d1b6b6151f8c03fdbd8a1b869)

src/core/service.c

@@ -3522,6 +3522,7 @@ 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) {