[MEDIUM] stats: add the "set maxconn" setting to the command line interface

This option permits to change the global maxconn setting within the
limit that was set by the initial value, which is now reported as the
hard maxconn value. This allows to immediately accept more concurrent
connections or to stop accepting new ones until the value passes below
the indicated setting.

The main use of this option is on systems where many haproxy instances
are loaded and admins need to re-adjust resource sharing at run time
to regain a bit of fairness between processes.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 98d021e..becdab7 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9561,6 +9561,14 @@
   delayed until the threshold is reached. The frontend might be specified by
   either its name or its numeric ID prefixed with a sharp ('#').
 
+set maxconn global <maxconn>
+  Dynamically change the global maxconn setting within the range defined by the
+  initial global maxconn setting. If it is increased and connections were
+  pending, they will immediately be accepted. If it is lowered to a value below
+  the current number of connections, new connections acceptation will be
+  delayed until the threshold is reached. A value of zero restores the initial
+  setting.
+
 set timeout cli <delay>
   Change the CLI interface timeout for current connection. This can be useful
   during long debugging sessions where the user needs to constantly inspect