[PATCH 6 of 8] mx_ops: add "open" callback

[Damien Riegel]

# HG changeset patch
# User Damien Riegel <damien.rie...@gmail.com>
# Date 1462473575 14400
#      Thu May 05 14:39:35 2016 -0400
# Node ID 9f66909d2d50d829c7828f997e0f26ff1a7bcdf9
# Parent  97603fb152bcbf331ffe73db9cf64733d558701b
mx_ops: add "open" callback

This is the callback that will be called to open a mailbox.

diff -r 97603fb152bc -r 9f66909d2d50 mutt.h
--- a/mutt.h    Thu May 05 15:09:34 2016 -0400
+++ b/mutt.h    Thu May 05 14:39:35 2016 -0400
@@ -872,6 +872,7 @@
 
 struct mx_ops
 {
+  int (*open)(struct _context *);
   int (*close)(struct _context *);
 };