add silence gen to wait apps

asterisk.conf's 'transmit_silence' option existed before
this patch, but was limited to only generating silence
while recording and sending DTMF.  Now enabling the
transmit_silence option generates silence during wait
times as well.

To achieve this, ast_safe_sleep has been modified to
generate silence anytime no other generators are present
and transmit_silence is enabled.  Wait apps not using
ast_safe_sleep now generate silence when transmit_silence
is enabled as well.

(closes issue #16524)
Reported by: kobaz

(closes issue #16523)
Reported by: kobaz
Tested by: dvossel

Review: https://reviewboard.asterisk.org/r/456/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@239712 65c4cc65-6c06-0410-ace0-fbb531ad65f3