gbproxy: Honour the BSS TLLI type when creating an SGSN TLLI

Currently gbproxy_make_sgsn_tlli always returns a foreign TLLI when
it uses the (SGSN) P-TMSI to generate one.

This patch changes the implementation to return a SGSN TLLI of the
same type like the BSS TLLI in that case.

Sponsored-by: On-Waves ehf
sgsn/test: Move message sending to send_0408_message

This replaces serveral occurences of duplicated code for message
creation and sending (passing to gsm0408_gprs_rcvmsg) into a single
function. In addition, the sgsn_tx_counter is always reset within
send_0408_message to simplify the code that checks for the number of
messages sent.

Sponsored-by: On-Waves ehf
sgsn/test: Add test_gmm_attach

This test checks the attach procedure until the Attach Complete is
received.

Note that authorization and GMM state updates are not working
properly yet.

Sponsored-by: On-Waves ehf
bsc: Move gsm_network_init function to libbsc

Currently libcommon depends on libbsc, because gsm_network_init
(libcommon/gsm_data.c) directly calls gsm_net_update_ctype
(libbsc/gsm_04_08_utils.c).

This patch moves gsm_network_init to a new file libbsc/net_init.c.

Sponsored-by: On-Waves ehf
bsc: Move gsm_subscriber_base.c to libcommon

Since it is planned to use struct gsm_subscriber to manage subscriber
data in the SGSN, this file which contains the generic subscriber
related methods is moved to libcommon.

Sponsored-by: On-Waves ehf