Add announce-to-first-user option for app_queue

In r386792, the ability to play prompts to the first caller in a call queue was
added. While this is arguably a bug fix for those who expect the first caller
to continue receiving prompts while the agent is dialed, it has the side effect
of preventing the first caller from hearing the agent immediately upon
bridging. This may not be a problem for those who really want this option, but
for those who didn't care whether or not the first caller in queue heard their
position, it was an issue.

This patch disables the ability for the first caller in the queue to hear
prompts and adds a new option, announce-to-first-user, to queues.conf. Those
who the behavior can enable it by setting this value to True.

Note that if we ever implement the ability to have the prompts be stopped
upon bridging, this option can be removed.

(closes issue ASTERISK-21782)
Reported by: Remi Quezada
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Merged revisions 391215 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 391241 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@391245 f38db490-d61c-443f-a65b-d21fe96a405b