MINOR: cli: implements 'reload' on master CLI

The reload command reload the haproxy master like it is done with a kill
-USR2 on the master process.
diff --git a/src/haproxy.c b/src/haproxy.c
index 20fc1de..994e39c 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -708,7 +708,7 @@
  * When called, this function reexec haproxy with -sf followed by current
  * children PIDs and possibly old children PIDs if they didn't leave yet.
  */
-static void mworker_reload()
+void mworker_reload()
 {
 	int next_argc = 0;
 	int j;