Fix VM snapshot handling for combined INBOX.

Fix VM snapshot handling for combined INBOX.

The snapshot API contains an option that allow for combining of new
and old messages within a single snapshot. New messages, however,
include options beyond just 'INBOX' - it also includes the Urgent
folder. A previous patch that combined INBOX and Urgent accidentally
impacted snapshots that attempted to gain messages from just the Old
folder. This patch fixes the snapshot gathering such that the API
returns the appropriate messages for the folder selected, with and
without the combine option.

This should make it more clear about what's happening.

Review: https://reviewboard.asterisk.org/r/2539/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@388816 65c4cc65-6c06-0410-ace0-fbb531ad65f3