Add ind_bitlen column to auc_3g to record each USIM's IND size according to
3GPP TS 33.102 -- default is 5 bits, as suggested by the spec.
Introduce auc_3g_ind to each connecting GSUP client to use as IND index for
generating auth tuples sent to this client.
With osmo_gsup_server_add_conn(), implement a scheme where clients receive
fixed auc_3g_ind indexes based on the order in which they connect; each new
connection takes the lowest unused auc_3g_ind, so in case one of the clients
restarts, it will most likely receive the same auc_3g_ind, and if one client
disconnects, no other clients' auc_3g_ind are affected.
Add gsup_server_test.c to test the auc_3g_ind index distribution scheme.
Depends: libosmocore I4eac5be0c0b2cede04464c4c3a0873102d952453 for llist_first
Related: OS#1969
Change-Id: If4501ed4ff8e923fa6fe8b80c44c5ad647a8ed60
