Iu: add UEA encryption

Add vty 'encryption uea 0 1 2', defaults to 'encryption uea 0' to yield
previous behavior.

If any UEA above 0 is enabled, include the UEA key in the Iu Security
Mode Command.

I noticed that only the code bit in st_iu_security_cmd_on_enter()
affects the test. The same code in gsm48_gmm_authorize() seems to be
dead code? But applying the patch there as well just to be safe.

We cannot yet verify the chosen UEA to match a configured UEA level,
because the iu_client.c does not send us message details with the
RANAP_IU_EVENT_SECURITY_MODE_COMPLETE.
Also we cannot yet send the set of configured UEA to the hNodeB, since,
again, iu_client.c does not provide the proper API for it.
The proper solution here is to completely dissolve iu_client.c and do
all Iu handling in osmo-sgsn itself -- osmo-msc has dropped iu_client.c
a long time ago. For now let's allow osmo-sgsn to use UEA, even though
we are not as strict or concise as we should be. It seems to me
osmo-sgsn still needs a lot of TLC to live up to Osmocom standards.

Related: SYS#5516
Related: I1a7c3b156830058c43f15f55883ea301d2d01d5f (osmo-ttcn3-hacks)
Change-Id: I27e8e0078c45426bf227bb44aac82a4875d18d0f
---
M include/osmocom/sgsn/sgsn.h
M src/sgsn/gprs_gmm.c
M src/sgsn/gprs_gmm_attach.c
M src/sgsn/gprs_ranap.c
M src/sgsn/sgsn_vty.c
5 files changed, 50 insertions(+), 3 deletions(-)