Use default 'alpha' value of 0

Since we don't know the RX and TX parameters of the BTS that might be used
with PCU, the MS should not adapt the TX power from the RX level. So the
MS should always transmits with same power.

Finding an 'alpha' and 'gamma' value that will result in a constant
RX level at the BTS is a task of deployment.
Add RSSI of data blocks to PCU interface

This bumps the PCU API version to 5 and requires osmo-bts >= 0.2.0.
Introduce new file for various measurements

The measurements include:
- DL bandwidth usage
- DL packet loss rate
- DL measurements by mobile
- UL measurements by BTS

In order to receive DL measurements from mobile, it must be enabled via
system information message at BSC.
Fix: Correctly display SGSN remote IP at debugging
Make the code handle SIGTERM.

This way the pcu can be easily stopped with a sysvinit script.