lan: unify DHCPv4/6 lease time timestamp format

odhcpd reports DHCPv6 lease time in seconds => meaning lease time duration. Which is unfortunately treated as unix timestamp in reForis Lan page (e.g. 1970-01-01 <some time>).

DHCPv4 lease time, on the other hand, is reported as timestamp of end of the lease (e.g. 2022-01-11 20:41).

Unify lease time format for both DHCPv4/6.

Edited Jan 11, 2023 by Martin Matějek

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