Permit a set of multiple different A5 ciphers

So far, the administrator had to pick one particular cipher which
would then be used throughout all subscribers/phones. This is a bit
impractical, as e.g. not all phones support A5/3.  Extend the VTY
command syntax in a backwards-compatible way to permit for multiple
ciphers.

NOTE: Like the previous code, OsmoMSC does *not yet check* whether
the configured cipher is compatible with the MS capabilities as
reported in CLASSMARK!  The network hence might choose an algorithm
not supported by the phone.  Fixing this is subject to another patch.

Closes: OS#2460
Change-Id: I79a4e2892eb5fbecc3d84e11dceffb7149db264b
diff --git a/src/libcommon-cs/common_cs.c b/src/libcommon-cs/common_cs.c
index 41c456e..170b62a 100644
--- a/src/libcommon-cs/common_cs.c
+++ b/src/libcommon-cs/common_cs.c
@@ -56,6 +56,8 @@
 
 	net->country_code = country_code;
 	net->network_code = network_code;
+	/* Permit a compile-time default of A5/3 and A5/1 */
+	net->a5_encryption_mask = (1 << 3) | (1 << 1);
 
 	/* Use 30 min periodic update interval as sane default */
 	net->t3212 = 5;