[PATCH] RFC - change GPRS cipher API

Hello.

Attached is a trivial patch which breaks existing GPRS cipher API of libosmocore by switching from fixed 64-bit length Kc to variable-length.

There are several justifications for that:

- compliance with ETSI TS 155.22 (GEA4 - 128 bits Kc) and all further versions - similarity to existing auth api (osmocom/crypt/auth.h uses 128 bits as well) - nobody uses this API anyway (except my other patches with GEA) - patch breaks nothing within libosmocore (make check succeeds) and openbsc (uses gea0 only)

That's why I think next libosmocore version should apply this patch and change unused API before someone actually start using it and makes transition more difficult.