Allow for transferer to retry when dialing an invalid extension.

Allow for transferer to retry when dialing an invalid extension.

This allows for a configurable number of attempts for a transferer
to dial an extension to transfer the call to. For Asterisk 13, the
default values are such that upgrading between versions will not
cause a behaivour change. For trunk, though, the defaults will be
changed to be more user-friendly.

Review: https://reviewboard.asterisk.org/r/4167
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Merged revisions 428145 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@428146 65c4cc65-6c06-0410-ace0-fbb531ad65f3