Enable adaptive jitter compensation
Enable adaptive jitter compensation

* check RTP socket parameter and enable necessary jitter buffering
* return the status of requested jitter buffering instead of always
  returning 0
* document function parameters and return value

Change-Id: I87d4d9fe990683adbb02b64cb39b78de533d8046

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis