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/manager.h

@@ -370,6 +370,8 @@ struct Manager {
         usec_t default_timer_accuracy_usec;
 
         OOMPolicy default_oom_policy;
+        int default_oom_score_adjust;
+        bool default_oom_score_adjust_set;
 
         int original_log_level;
         LogTarget original_log_target;