gbproxy: Add global and per peer counters

This adds counters that are incremented when errors are detected.
It also modifies the VTY command 'show gbproxy' so that
'show gbproxy stats' shows the counters.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gbproxy/gbproxy_test.ok b/openbsc/tests/gbproxy/gbproxy_test.ok
index b8920ac..e4b7aba 100644
--- a/openbsc/tests/gbproxy/gbproxy_test.ok
+++ b/openbsc/tests/gbproxy/gbproxy_test.ok
@@ -746,7 +746,9 @@
 Peers:
   NSEI 4096, BVCI 4114, not blocked, RAC 10-32-16464-96
   NSEI 8192, BVCI 8194, not blocked, RAC 10-32-16464-96
+    NSEI mismatch                   : 1
   NSEI 4096, BVCI 4098, not blocked, RAC 10-32-16464-96
+    NSEI mismatch                   : 1
 PROCESSING BVC_RESET_ACK from 0x05060708:32000
 00 00 00 00 23 04 82 10 02