gbproxy: Make STATUS messages spec compliant

Currently the gbproxy sends STATUS messages that are not compliant to
GSM 08.18, 10.4.14.1: The BVCI must be included if (and only if) the
cause is either "BVCI blocked" or "BVCI unknown".

This patch adds a missing BVCI to UNKNOWN_BVCI and BVCI_BLOCKED
status messages if the BVCI is available. Otherwise, INV_MAND_INF is
used instead.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index fc767a4..1cf3bb0 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -1868,7 +1868,7 @@
 
 NS UNITDATA MESSAGE to SGSN, BVCI 0x0000, msg length 24 (gprs_ns_sendmsg)
 MESSAGE to SGSN at 0x05060708:32000, msg length 28
-00 00 00 00 41 07 81 05 15 92 0c 1f 84 cc d1 75 8b 1b 86 00 f1 99 00 63 60 1d 81 01 
+00 00 00 00 41 07 81 21 15 92 0c 1f 84 cc d1 75 8b 1b 86 00 f1 99 00 63 60 1d 81 01 
 
 result (BVC_SUSPEND_ACK) = 28
 
@@ -2307,7 +2307,7 @@
 
 NS UNITDATA MESSAGE to SGSN, BVCI 0x0000, msg length 24 (gprs_ns_sendmsg)
 MESSAGE to SGSN at 0x05060708:32000, msg length 28
-00 00 00 00 41 07 81 05 15 92 0c 1f 84 ef e2 b7 00 1b 86 00 f1 99 00 63 60 1d 81 01 
+00 00 00 00 41 07 81 21 15 92 0c 1f 84 ef e2 b7 00 1b 86 00 f1 99 00 63 60 1d 81 01 
 
 result (BVC_SUSPEND_ACK) = 28
 
@@ -2816,7 +2816,7 @@
 
 NS UNITDATA MESSAGE to SGSN, BVCI 0x0000, msg length 24 (gprs_ns_sendmsg)
 MESSAGE to SGSN at 0x05060708:32000, msg length 28
-00 00 00 00 41 07 81 05 15 92 0c 1f 84 ef e2 b7 00 1b 86 00 f1 99 00 63 60 1d 81 01 
+00 00 00 00 41 07 81 21 15 92 0c 1f 84 ef e2 b7 00 1b 86 00 f1 99 00 63 60 1d 81 01 
 
 result (BVC_SUSPEND_ACK) = 28