TRX: add Uplink DTX support for FR/HR
TRX: add Uplink DTX support for FR/HR

* detect SID and set RTP Marker accordingly (emulate ONSET events)
* set proper FN in TCH_IND
* detect speech pause and do not send dummy 'bad' frames during that
  time

Change-Id: Id518e5c667df7773c281effb9e75b66bf898f6fc
Related: OS#1750

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts