Allow configuration of minsecs and nextaftercmd per mailbox.

Previously only configurable globally. A unit test has also been written to 
provide protection against parse failures for supported mailbox options.

(closes issue #16864)
Reported by: kobaz
Patches: 
      voicemail2.patch uploaded by kobaz (license 834)

Review: https://reviewboard.asterisk.org/r/555/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@254321 65c4cc65-6c06-0410-ace0-fbb531ad65f3