[MINOR] Allow shutdown of sessions when a server becomes unavailable

This adds the "on-marked-down shutdown-sessions" statement on "server" lines,
which causes all sessions established on a server to be killed at once when
the server goes down. The task's priority is reniced to the highest value
(1024) so that servers holding many tasks don't cause a massive slowdown due
to the wakeup storm.
diff --git a/include/types/server.h b/include/types/server.h
index cf0a0df..1a9d60d 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -125,6 +125,7 @@
 	int rise, fall;				/* time in iterations */
 	int consecutive_errors_limit;		/* number of consecutive errors that triggers an event */
 	short observe, onerror;			/* observing mode: one of HANA_OBS_*; what to do on error: on of ANA_ONERR_* */
+	short onmarkeddown;			/* what to do when marked down: on of HANA_ONMARKEDDOWN_* */
 	int inter, fastinter, downinter;	/* checks: time in milliseconds */
 	int slowstart;				/* slowstart time in seconds (ms in the conf) */
 	int result;				/* health-check result : SRV_CHK_* */