app_chanspy: Add 'D' option for dual-channel audio

app_chanspy: Add 'D' option for dual-channel audio

Adds the 'D' option to app chanspy that causes the input and output
frames of the spied channel to be interleaved in the spy output frame.
This allows the input and output of the spied channel to be decoded
separately by the receiver.

If the 'o' option is also set, the 'D' option is ignored as the
audio being spied is inherently one direction.

Fixes: #569
UserNote: The ChanSpy application now accepts the 'D' option which
will interleave the spied audio within the outgoing frames. The
purpose of this is to allow the audio to be read as a Dual channel
stream with separate incoming and outgoing audio. Setting both the
'o' option and the 'D' option and results in the 'D' option being
ignored.