core: add a new setting DefaultOOMScoreAdjust= and set it to 100 above service manager's by default

Let's make our service managers slightly less likely to be killed by the
OOM killer by adjusting our services' OOM score adjustment to 100 above
ours. Do this conservatively, i.e. only for regular user sessions.

src/core/unit.c

@@ -187,6 +187,11 @@ static void unit_init(Unit *u) {
         if (ec) {
                 exec_context_init(ec);
 
+                if (u->manager->default_oom_score_adjust_set) {
+                        ec->oom_score_adjust = u->manager->default_oom_score_adjust;
+                        ec->oom_score_adjust_set = true;
+                }
+
                 if (MANAGER_IS_SYSTEM(u->manager))
                         ec->keyring_mode = EXEC_KEYRING_SHARED;
                 else {