04.08: apply new bitmask functions, fix bitmask use

Replace hardcoded protocol discriminator and message type bitmasks with
function calls recently introduced in libosmocore.

Note that the release 98 bitmasks slightly differ from the release 99 bitmasks.
This patch uses the "default" gsm48_hdr_msg_type invocation, thus it depends on
libosmocore whether 98 or 99 bitmasks are used.

In some places, use of the bitmask was erratic. Fix these implicitly by
employing the bitmask functions:

 * silent_call.c: silent_call_reroute(): add missing bitmask for MM.
 * bsc_msg_filter.c: bsc_msg_filter_initial(): RR vs. MM messages.
 * osmo_bsc_filter.c: bsc_find_msc() and bsc_scan_bts_msg(): RR vs. MM
   messages.
 * bsc_nat_rewrite.c: bsc_nat_rewrite_msg(): SMS vs. CC messages.
 * bsc_ussd.c: no bitmask is applicable for the message types used here.
 * gb_proxy.c: gbproxy_imsi_acquisition(): missing bit mask for pdisc.

In gprs_gb_parse.c: gprs_gb_parse_dtap(), add a log notice for unexpected
message types.
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index 9551335..6e6b03b 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -462,8 +462,8 @@
 	if (link_info->imsi_acq_pending && link_info->imsi_len > 0) {
 		int is_ident_resp =
 			parse_ctx->g48_hdr &&
-			parse_ctx->g48_hdr->proto_discr == GSM48_PDISC_MM_GPRS &&
-			parse_ctx->g48_hdr->msg_type == GSM48_MT_GMM_ID_RESP;
+			gsm48_hdr_pdisc(parse_ctx->g48_hdr) == GSM48_PDISC_MM_GPRS &&
+			gsm48_hdr_msg_type(parse_ctx->g48_hdr) == GSM48_MT_GMM_ID_RESP;
 
 		/* The IMSI is now available */
 		gbproxy_flush_stored_messages(peer, msg, now, link_info,