db: extend database schema to support 256bit K and/or OP[c] values

Other UMTS AKA algorithms than MILENAGE (notably TUAK) support K sizes
of up to 256bit, or mandate a OP/OPc size of 256 bit.

Let's extend our database schema to accommodate such larger sizes.

Change-Id: Ibbde68484c904507a15c35cbfdf88cd47d0c7039
diff --git a/tests/test_subscriber.vty b/tests/test_subscriber.vty
index dbe9327..9036fb8 100644
--- a/tests/test_subscriber.vty
+++ b/tests/test_subscriber.vty
@@ -275,14 +275,14 @@
   k  Set Encryption Key K
 
 OsmoHLR# subscriber imsi 123456789023000 update aud3g milenage k ?
-  K  K as 32 hexadecimal characters
+  K  K as 32/64 hexadecimal characters
 
 OsmoHLR# subscriber imsi 123456789023000 update aud3g milenage k Deaf0ff1ceD0d0DabbedD1ced1ceF00d ?
   op   Set OP key
   opc  Set OPC key
 
 OsmoHLR# subscriber imsi 123456789023000 update aud3g milenage k Deaf0ff1ceD0d0DabbedD1ced1ceF00d opc ?
-  OP_C  OP or OPC as 32 hexadecimal characters
+  OP_C  OP or OPC as 32/64 hexadecimal characters
 
 OsmoHLR# subscriber imsi 123456789023000 update aud3g milenage k Deaf0ff1ceD0d0DabbedD1ced1ceF00d opc CededEffacedAceFacedBadFadedBeef ?
   [ind-bitlen]  Set IND bit length