fix inter-BSC-in handover encryption

In the field we saw Handover Requests without any Chosen Encryption
Algorithm IE, and osmo-bsc completely failed on those. This made me
understand my mistake from when I wrote this handover code.

So far, from a BSSMAP Handover Request, we (I) used only the Chosen
Encryption Algorithm IE to pick the encryption to use on the target
lchan. That is very wrong.

Instead, figure out the intersection of permitted algorithms MSC & BSC,
and pick the best of those. Which means, actually, completely ignore the
Chosen Encryption Algorithm IE.

In the message, the permitted algorithms are passed as a bitmask. The
current code using gsm0808_dec_encrypt_info() passes this on as an
array. In order to select_best_cipher(), I could convert that array back
to a bitmask. Instead pass the bitmask on from message decoding
alongside the struct gsm0808_encrypt_info in req->ei_as_bitmask.

Related: SYS#5839
Change-Id: Iffedc981b60d309ed2e5decd5efedee07a757b53
---
M include/osmocom/bsc/gsm_data.h
M src/osmo-bsc/handover_fsm.c
M src/osmo-bsc/osmo_bsc_bssap.c
3 files changed, 25 insertions(+), 7 deletions(-)