MINOR: stats: add check_desc and agent_desc to the output fields

Added these two new fields to the CSV output :
- check_desc : short human-readable description of check_status
- agent_desc : short human-readable description of agent_status

Also factor two tests for enabled checks.
diff --git a/doc/management.txt b/doc/management.txt
index 46b03aa..6b491ff 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1014,6 +1014,8 @@
         L7STS   -> agent reported "fail", "stop", or "down"
  63. agent_code [...S]: numeric code reported by agent if any (unused for now)
  64. agent_duration [...S]: time in ms taken to finish last check
+ 65. check_desc [...S]: short human-readable description of check_status
+ 66. agent_desc [...S]: short human-readable description of agent_status
 
 
 9.2. Unix Socket commands