MINOR: cli: change a server health check port through the stats socket

Introduction of a new CLI command "set server <srv> check-port <port>' to
allow admins to change a server's health check port at run time.

This changes the equivalent of the configuration server parameter
called 'port'.
diff --git a/doc/management.txt b/doc/management.txt
index f559ffc..d4670d5 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1595,6 +1595,9 @@
   switch a server's state regardless of some slow health checks for example.
   Note that the change is propagated to tracking servers if any.
 
+set server <backend>/<server> check-port <port>
+  Change the port used for health checking to <port>
+
 set server <backend>/<server> state [ ready | drain | maint ]
   Force a server's administrative state to a new state. This can be useful to
   disable load balancing and/or any traffic to a server. Setting the state to