[MEDIUM] Health check reporting code rework + health logging, v3

This patch adds health logging so it possible to check what
was happening before a crash. Failed healt checks are logged if
server is UP and succeeded healt checks if server is DOWN,
so the amount of additional information is limited.

I also reworked the code a little:

 - check_status_description[] and check_status_info[] is now
   joined into check_statuses[]

 - set_server_check_status updates not only s->check_status and
   s->check_duration but also s->result making the code simpler

Changes in v3:
 - for now calculate and use local versions of health/rise/fall/state,
   it is a slow path, no harm should be done. One day we may centralize
   processing of the checks and remove the duplicated code.
 - also log checks that are restoring current state
 - use "conditionally succeeded" for 404 with disable-on-404
diff --git a/doc/configuration.txt b/doc/configuration.txt
index e455368..2055a09 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -731,6 +731,8 @@
 [no] option http_proxy      X          X         X         X
 [no] option log-separate-
             errors          X          X         X         -
+[no] option log-health-     X          -         X         X
+            checks
 [no] option logasap         X          X         X         -
 [no] option nolinger        X          X         X         X
 option originalto           X          X         X         X
@@ -2464,6 +2466,23 @@
              logging.
 
 
+log-health-checks
+no log-health-checks
+  Enable or disable logging of health checks
+  May be used in sections :   defaults | frontend | listen | backend
+                                 yes   |    no    |   yes  |  yes
+  Arguments : none
+
+  Enable health checks logging so it possible to check for example what
+  was happening before a server crash. Failed health check are logged if
+  server is UP and succeeded health checks if server is DOWN, so the amount
+  of additional information is limited.
+
+  If health check logging is enabled no health check status is printed
+  when servers is set up UP/DOWN/ENABLED/DISABLED.
+
+  See also: "log" and section 8 about logging.
+
 option logasap
 no option logasap
   Enable or disable early logging of HTTP requests