Resolve "default preferred-lifetime"

Closes #2835 (closed)

Remove the preferred-lifetime, global default, of 3600 seconds so it is unspecified add add a check that replaces the outbound value with 0.625 * valid-lifetime when it is unspecified or greater than valid-lifetime. This makes default behavior match ISC DHCP but goes step further by not allowing users to send an insane value that per RFC clients should reject.

Update the ARM accordingly.