MINOR: cli: new stats socket command: show backend

new stats socket command which displays only the list of backends
available in the current process.
For now only the backend name is displayed.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 671a4c2..aa00628 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -15794,6 +15794,9 @@
     is the slash ('/') in header name "header/bizarre", which is not a valid
     HTTP character for a header name.
 
+show backend
+  Dump the list of backends available in the running process
+
 show info
   Dump info about haproxy status on current process.