Store the offset in the teletone buffer in the b-channel private data.

An NT-mode setup with ftmod_misdn showed severe (dial-)tone distortions
in a sound editor (330Hz sine wave phase errors), caused by
using a global teletone buffer offset.

Switching to a per-channel offset, that is advanced by the amount
of data actually written to the channel, removes (almost) all
distortions.

There is still a minimal phase error every ~500ms (audible) that
needs more investigating.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>