Fix #1024 by changing decode channel assigment to PDs

When assigning the decoder stack channels to the libsrd
instance's channels, channels that had no signal assigned
to them were still assigned anyway. This patch fixes this bug.

After doing this, another subtle bug became apparent:
The mapping between channels and bits in the data stream sent
to the PD was done via DecodeChannel->id. This is however
insufficient as the channels of the decoder stack have
IDs that may or may not match the ID needed for the data
stream. Example:

A PD has 4 channels: A, B, C and D. In PV, those channels
have the IDs 0, 1, 2 and 3. If the user only assigns A and D,
Decoder::create_decoder_inst() will use the IDs 0 and 3 as
the bit positions of those signals in the data stream sent
to libsrd. This is obviously wrong.

Hence, we now use a separate bit_id for this purpose.