MINOR: haproxy: Add `-cc` argument

This patch adds the `-cc` (check condition) argument to evaluate conditions on
startup and return the result as the exit code.

As an example this can be used to easily check HAProxy's version in scripts:

    haproxy -cc 'version_atleast(2.4)'

This resolves GitHub issue #1246.

Co-authored-by: Tim Duesterhus <tim@bastelstu.be>
diff --git a/doc/management.txt b/doc/management.txt
index b1ef55e..39c3a31 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -195,6 +195,10 @@
     to bind. The exit status is zero if everything is OK, or non-zero if an
     error is encountered. Presence of warnings will be reported if any.
 
+  -cc : evaluates a condition as used within a conditional block of the
+    configuration. The exit status is zero if the condition is true, 1 if the
+    condition is false or 2 if an error is encountered.
+
   -d : enable debug mode. This disables daemon mode, forces the process to stay
     in foreground and to show incoming and outgoing events. It must never be
     used in an init script.