config: make config_dir absolute

This makes it easier for DB drivers and plugins

lightningd/options.c

@@ -860,6 +860,8 @@ static void handle_minimal_config_opts(struct lightningd *ld,
 			       &ld->config_dir,
 			       "Set working directory. All other files are relative to this");
 
+	ld->config_dir = path_join(ld, path_cwd(tmpctx), take(ld->config_dir));
+
 	ld->wallet_dsn = tal_fmt(ld, "sqlite3://%s/lightningd.sqlite3", ld->config_dir);
 	opt_register_early_arg("--wallet", opt_set_talstr, NULL,
 			       &ld->wallet_dsn,