IMAP storage functioned under the assumption that folders

IMAP storage functioned under the assumption that folders
such as "Work" and "Family" would be subfolders of the
INBOX. This is an invalid assumption to make, but it could
be desirable to set up folders in this manner, so a new
option for voicemail.conf, "imapparentfolder" has been
added to allow for this.

(closes issue #13142)
Reported by: jaroth
Patches:
      parentfolder.patch uploaded by jaroth (license 50)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@135067 65c4cc65-6c06-0410-ace0-fbb531ad65f3