CLEANUP: config: Return ERR_NONE from config callbacks instead of 0

Return ERR_NONE instead of 0 on success for all config callbacks that should
return ERR_* codes. There is no change because ERR_NONE is a macro equals to
0. But this makes the return value more explicit.
diff --git a/src/tcpcheck.c b/src/tcpcheck.c
index 75d97f3..3ecb0ba 100644
--- a/src/tcpcheck.c
+++ b/src/tcpcheck.c
@@ -3470,7 +3470,7 @@
 	struct tcpcheck_rule *chk, *back;
 	char *comment = NULL, *errmsg = NULL;
 	enum tcpcheck_rule_type prev_action = TCPCHK_ACT_COMMENT;
-	int ret = 0;
+	int ret = ERR_NONE;
 
 	if (!(px->cap & PR_CAP_BE) || (px->options2 & PR_O2_CHK_ANY) != PR_O2_TCPCHK_CHK) {
 		deinit_proxy_tcpcheck(px);