Introduce support for XOR-2G algorithm

So far we supported a "xor" algorithm in osmo-hlr, without specifying
whether it's the XOR-3G or the (different) XOR-2G algorithm.

Furthermore, it was buggy in the sense that it permitted the XOR[-3G]
for 2G authentication data in the database.

This patch
* renames existing "xor" to "xor-3g"
* disallows "xor-3g" usage with 2G authentication data
* introduces support for XOR-2G as "xor-2g" in the VTY

Change-Id: I039a1f84fda54a908a82fe621e7fd078cb85e4c6
---
M include/osmocom/hlr/hlr_vty.h
M src/db_hlr.c
M src/hlr_vty_subscr.c
M tests/auc/auc_test.c
M tests/db/db_test.c
M tests/db/db_test.err
M tests/db_upgrade/create_subscribers.vty
M tests/db_upgrade/db_upgrade_test.ok
M tests/db_upgrade/hlr_db_v0.sql
M tests/test_subscriber.ctrl
M tests/test_subscriber.vty
M tests/test_subscriber_errors.ctrl
12 files changed, 62 insertions(+), 36 deletions(-)