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

The expiry time in a dnsmasq lease file line may be 0 (i.e.
expiry date = 01/01/1970 00:00:00 GMT) 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. This is to mimic the odhcp code below.
If the expiry date is not equal to 0, then just do exactly what was
done before (return the os.diff of current time and ts).

Signed-off-by: Cody R. Brown <dev@codybrown.ca>