[MEDIUM] Decrease server health based on http responses / events, version 3

Implement decreasing health based on observing communication between
HAProxy and servers.

Changes in this version 2:
 - documentation
 - close race between a started check and health analysis event
 - don't force fastinter if it is not set
 - better names for options
 - layer4 support

Changes in this version 3:
 - add stats
 - port to the current 1.4 tree
diff --git a/include/proto/checks.h b/include/proto/checks.h
index bd70164..e3fe7ac 100644
--- a/include/proto/checks.h
+++ b/include/proto/checks.h
@@ -29,6 +29,7 @@
 const char *get_check_status_info(short check_status);
 struct task *process_chk(struct task *t);
 int start_checks();
+void health_adjust(struct server *s, short status);
 
 #endif /* _PROTO_CHECKS_H */