all: update handling of local config

For local config, we don't want to alter what user explicitly set, and
only try filling in missing value.

While at it, also remove the dnsmasq port delete on openwrt, we don't
need that hack anymore.

internal/router/openwrt/openwrt.go

@@ -49,11 +49,6 @@ func (o *Openwrt) PreRun() error {
 }
 
 func (o *Openwrt) Setup() error {
-	// Delete dnsmasq port if set.
-	if _, err := uci("delete", "dhcp.@dnsmasq[0].port"); err != nil && !errors.Is(err, errUCIEntryNotFound) {
-		return err
-	}
-
 	data, err := dnsmasq.ConfTmpl(dnsmasq.ConfigContentTmpl, o.cfg)
 	if err != nil {
 		return err