SIP: Allow early audio on incomming connections at SIP interface

In order to provide internal tones, a clock is used to generate
chunks of 160 samples. If no tones are provided and if audio is
bridged, it is forwarded as usual.

In order to provide early audio on SIP trunk, "tones yes" must be set
at interface.conf.
In order to receive early audio from SIP trunk, "earlyb yes" must be
set at interface.conf.