luci-base: luci.tools.status: properly parse infinite odhcpd leases

The expiry time in an odhcpd lease file line may be -1 to denote an infinite
lease time, so adjust the code to properly support that.

The expiry attribute of the lease object will be set to "false" in case of an
infinite lease.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>