MEDIUM: server: adding support for check_port in server state

We can currently change the check-port using the cli command `set server
check-port` but there is a consistency issue when using server state.
This patch aims to fix this problem but will be also a good preparation
work to get rid of checkport flag, so we are able to know when checkport
was set by config.

I am fully aware this is not making github #953 moving forward, I
however think this might be acceptable while waiting for a proper
solution and resolve consistency problem faced with port settings.

Signed-off-by: William Dauchy <wdauchy@gmail.com>
diff --git a/doc/management.txt b/doc/management.txt
index de3fbf6..b74aba7 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -2447,6 +2447,7 @@
      srv_port:                    Server port.
      srvrecord:                   DNS SRV record associated to this SRV.
      srv_use_ssl:                 use ssl for server connections.
+     srv_check_port:              Server check port.
 
 show sess
   Dump all known sessions. Avoid doing this on slow connections as this can