Let vm_mailbox_snapshot_create's combine option apply to "Urgent" as well

Let vm_mailbox_snapshot_create's combine option apply to "Urgent" as well

The vm_mailbox_snapshot_create function has an option that combines the
contents of INBOX and Old into a single snapshot. The intent of this is that
both 'new' messages and 'deleted' messages are given in a single snapshot, as
some applications prefer this view of the voicemail world. Unfortunately, the
initial implementation ignored the "Urgent" folder. The "Urgent" folder is a
pseudo-INBOX, in that new messages left with the 'U' flag will be placed in
that folder as opposed to INBOX. Thus, the option failed the intent with which
it was added.

This patch makes it so that the "Urgent" folder is included in the snapshot
when that option is used.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@381835 65c4cc65-6c06-0410-ace0-fbb531ad65f3