gbproxy: Use a separate regexp for routing

Currently one regexp ('patching') is used for all matching.

This patch adds a second category 'routing' which is exclusively used
for SGSN selection. It also adds a corresponding VTY command:

  - match-imsi patching RE : MS related patching (currently APN)
  - match-imsi routing RE  : Select secondary SGSN on match only
  - no match-imsi          : Clear all filter expressions

Ticket: OW#1258
Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index 41ff4a7..c23e718 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -584,7 +584,7 @@
 	if (link_info && cfg->route_to_sgsn2) {
 		if (cfg->acquire_imsi && link_info->imsi_len == 0)
 			sgsn_nsei = 0xffff;
-		else if (gbproxy_imsi_matches(cfg, GBPROX_MATCH_PATCHING,
+		else if (gbproxy_imsi_matches(cfg, GBPROX_MATCH_ROUTING,
 					      link_info))
 			sgsn_nsei = cfg->nsip_sgsn2_nsei;
 	}