ctrl: Add command to get the current load of a BTS

Add a command and test to see the current channel load and
available channels per BTS.

Related: SYS#798
ctrl: Allow to query if the OML link is connected or not

Related: SYS#798
msc: Add net back pointer to gsm_trans

Currently the net pointer is obtained from trans->subscr->net. On the
other hand, the list gsm_trans object is managed by the net object.

This patch adds the back pointer to the structure and replaces all
trans->subscr->net by trans->net expressions. In trans_alloc() the
trans->net pointer is obtained from the subscr object.

Sponsored-by: On-Waves ehf
msc: Don't use the subscriber to access the net object

Sponsored-by: On-Waves ehf
msc: Add and use gsm_subscriber_group

Currently every subcriber object directly refers to the gsm_network
which contains a flag shared by every related subscriber
(keep_subscr). This adds a dependency on gsm_network even if only the
function defined in gsm_subscriber_base.c are used.

This patch adds a new struct gsm_subscriber_group which contains the
keep_subscr flag and a back reference to the network object. The
latter is not dereferenced in gsm_subscriber_base.c, so it can safely
be set to NULL when only that part of the gsm_subscriber API is being
used. It also changes that API to use gsm_subscriber_group instead of
gsm_network parameters.

Since there are some places where a pointer to the gsm_network is
needed but where only a gsm_subscriber is available, a 'net' back
pointer is added to the group struct, too. Nevertheless subscr group
and network could be separated completely, but this is not the topic
of this commit.

Sponsored-by: On-Waves ehf
bsc/test: Add tests for gsm_subscriber base

This commit adds test for the generic part of gsm_subscriber like
reference counting and flag usage.

Sponsored-by: On-Waves ehf