dhcpd,dumpleases: write and use 64-bit current time in lease file.

dhcpd,dumpleases: write and use 64-bit current time in lease file.
 without it, determination of remaining lease time is unreliable.
 Yes, it's costly in code size, but without it, dumpleases output
 is misleading!

function                                             old     new   delta
read_leases                                          143     282    +139
dumpleases_main                                      447     572    +125
write_leases                                         193     233     +40
hton64                                                29      58     +29
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