ubus: improve correspondence between DHCPv[46]

ubus: improve correspondence between DHCPv[46]

The same parameter is called "accept-reconf-nonce" in the "ipv4leases" case and
"accept-reconf" in the "ipv6leases". I couldn't find anything in the OpenWrt
trees which depended on either naming, but renaming the IPv4 case seems to be
the safer bet since it is not part of the standard installation.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/267
Signed-off-by: Álvaro Fernández Rojas <[email protected]>