[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 58de0881dedec01e9dedcfa14c2fbb48ed6135db
# Parent  d68a087328c13219f53944a46fad4aa8db1bc458
mx_ops: add "open" callback

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

diff -r d68a087328c1 -r 58de0881dede 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 *);
 };