login: fix session_kill(..., KILL_LEADER,...) (#35105)

`loginctl kill-session --kill-whom=leader <N>` (or the D-Bus equivalent)
doesn't work because logind ends up calling `KillUnit(..., "main", ...)`
on a scope unit and these don't have a `MainPID` property. Here, I just
make it send a signal to the `Leader` directly.

(cherry picked from commit 8254755)
(cherry picked from commit c89c5d0)

Author: 12paper

src/login/logind-session.c

@@ -1336,10 +1336,20 @@ SessionState session_get_state(Session *s) {
 int session_kill(Session *s, KillWho who, int signo) {
         assert(s);
 
-        if (!s->scope)
-                return -ESRCH;
+        switch (who) {
+
+        case KILL_ALL:
+                if (!s->scope)
+                        return -ESRCH;
+
+                return manager_kill_unit(s->manager, s->scope, KILL_ALL, signo, NULL);
 
-        return manager_kill_unit(s->manager, s->scope, who, signo, NULL);
+        case KILL_LEADER:
+                return pidref_kill(&s->leader, signo);
+
+        default:
+                assert_not_reached();
+        }
 }
 
 static int session_open_vt(Session *s, bool reopen) {