make it optional where the hlr database is stored

Add --database to define where the database is stored. The default
was changed to not store the file in /tmp anymore.
diff --git a/include/openbsc/db.h b/include/openbsc/db.h
index e02bb62..df703e0 100644
--- a/include/openbsc/db.h
+++ b/include/openbsc/db.h
@@ -24,7 +24,7 @@
 
 #include <openbsc/gsm_subscriber.h>
 
-int db_init();
+int db_init(const char *name);
 int db_prepare();
 int db_fini();