sgsn: Add support for authentication triplets

This commit add data structures, functions, initialization, and VTY
commands for per subscriber authentication triplets.

The following VTY command is added:

  - update-subscriber imsi IMSI \
    insert auth-triplet <1-5> sres SRES rand RAND kc KC

Note that the triplets are not really used by the SGSN yet.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/sgsn/sgsn_test.ok b/openbsc/tests/sgsn/sgsn_test.ok
index 7d739ad..eff47c4 100644
--- a/openbsc/tests/sgsn/sgsn_test.ok
+++ b/openbsc/tests/sgsn/sgsn_test.ok
@@ -1,5 +1,6 @@
 Testing LLME allocations
 Testing core subscriber data API
+Testing authentication triplet handling
 Testing GMM detach
 Testing GMM detach (power off)
 Testing GMM detach (no MMCTX)