gprs: Create a gprs_utils file and move to be shared code in there

We intend to move some of these routines to libosmocore but to avoid
a feature symbol clash we are prefixing these routines with gprs_.

Done with Jacob
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index 0be2519..29494c6 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -32,6 +32,7 @@
 
 #include <openbsc/debug.h>
 #include <openbsc/gb_proxy.h>
+#include <openbsc/gprs_utils.h>
 #include <openbsc/vty.h>
 
 #include <osmocom/vty/command.h>
@@ -92,7 +93,7 @@
 	       if (g_cfg->core_apn_size > 0) {
 		       char str[500] = {0};
 		       vty_out(vty, " core-access-point-name %s",
-			       gbprox_apn_to_str(str, g_cfg->core_apn,
+			       gprs_apn_to_str(str, g_cfg->core_apn,
 						 g_cfg->core_apn_size));
 	       } else {
 		       vty_out(vty, " core-access-point-name none");
@@ -229,7 +230,7 @@
 		g_cfg->core_apn =
 			talloc_realloc_size(NULL, g_cfg->core_apn, apn_len + 1);
 		g_cfg->core_apn_size =
-			gbprox_str_to_apn(g_cfg->core_apn, apn, apn_len + 1);
+			gprs_str_to_apn(g_cfg->core_apn, apn, apn_len + 1);
 	}
 
 	return CMD_SUCCESS;