FEC recalculation always reports errors for null AMBE frames, and
there is no DTMF present by definition.
In practice, null AMBE data is often (always?) sent by the Kenwood
TH-D74 in the first 21 voice frames, and I've also seen it at the end
of a fast data transmission.
This commit adds a maybeFixupVoiceFrame() function that is used by
both the RF and Net code to manage FEC regeneration and DTMF blanking
in voice frames.
The presence of Fast Data is discovered by reading the mini-header in
every second data frame. If found, FEC regeneration and DTMF blanking
are disabled for the current and next voice frames.
An exception is voice frames that have a sync frame instead of a data
frame. This commit always disables FEC regeneration and DTMF blanking
for these frames. A later commit will add support for these frames by
setting aside the voice frame until the next data frame can be read.
This commit also includes a number of debugging statements that will
be removed in a later commit.
