Rename RFCOMM_GetFrameDataLength() to RFCOMM_GetVariableFieldValue() as it is...

Rename RFCOMM_GetFrameDataLength() to RFCOMM_GetVariableFieldValue() as it is multi-purpose, and modify it to move the buffer pointer itself rather than relying on the caller.

Make RFCOMM_GetChannelData() return NULL if the required channel DLCI is found, but the channel is closed.

Add modem signals (BREAK, DCD, etc.) to the channel state array, and add in a handler for the Modem Status RFCOMM control command.