[nat] Make the string -> regexp parsing public

This way it can be used from within a test case to test
the regexps..
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 6baa952..275ecb4 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -272,3 +272,17 @@
 		return 0;
 	}
 }
+
+void bsc_parse_reg(void *ctx, regex_t *reg, char **imsi, int argc, const char **argv)
+{
+	if (*imsi) {
+		talloc_free(*imsi);
+		*imsi = NULL;
+	}
+	regfree(reg);
+
+	if (argc > 0) {
+		*imsi = talloc_strdup(ctx, argv[0]);
+		regcomp(reg, argv[0], 0);
+	}
+}