MEDIUM: stats: report the listeners' address in the CSV output

It's the same principle as for the server dump, and we use this field
for the HTML dump of course.
diff --git a/doc/management.txt b/doc/management.txt
index 9c07151..8ce8e6a 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1022,7 +1022,7 @@
  70. agent_rise [...S]: agent's "rise" parameter, normally 1
  71. agent_fall [...S]: agent's "fall" parameter, normally 1
  72. agent_health [...S]: agent's health parameter, between 0 and rise+fall-1
- 73. addr [...S]: server's address:port. IPv6 has brackets around the address.
+ 73. addr [L..S]: address:port or "unix". IPv6 has brackets around the address.
  74: cookie [..BS]: server's cookie value or backend's cookie name