Added functions:
- gprs_bssgp_pcu_rx_paging_ps() for handling paging message from BSSGP;
- write_paging_request() for writing paging request message;
- gprs_rlcmac_paging_request() and pcu_l1if_tx_pch() for sending paging request message to BTS.
The OpenBTS socket functions are moved from main to pcu_l1_if.cpp.
New sysmo_l1_if.cpp is introduced. It used special unix socket interface
to connect to sysmo-BTS. This is required to access CCCH/RACH and info
about cell layout. Traffic is also forwarded via this interface, but
it direct access of L1 baseband DSP will be added soon.
In order to handle ready-to-send requests above l1_if, the transmit
queue (for downlink blocks) is moved to gprs_rlcmac.cpp.
The TBF instance additionally holds TRX and TS info, but this is only
a hack currently. TBF instance requires more details about allocated
ressources in the future.