Added Confbridge record_file_append option.

Currently, if one starts, stops, and then starts a recording again for a
conference the recorded data is appended to the file originally created
on the first record start.  An option record_file_append has been added
that defaults to "yes", but when set to "no" will force creation of a new
file between every record start/stop.

(issue AST-1088)
Reported by: John Bigelow
Review: http://reviewboard.digium.internal/r/374/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381729 65c4cc65-6c06-0410-ace0-fbb531ad65f3