netifd: Request DHCP option 121 (classless route) by default

netifd: Request DHCP option 121 (classless route) by default

This option, defined by RFC3442, allows a DHCP server to send static
routes to a client.  But the client has to request this option
explicitely.

Static routes are useful when the gateway configured by DHCP cannot be
in the same subnet as the client.  This happens, for instance, when
using DHCP to hand out addresses in /32 subnets.

A new configuration option "classlessroute" is available, allowing
users to disable this feature (the option defaults to true).

Other DHCP clients already request this option by default (dhcpcd, for
instance, and possibly Windows).  If a DHCP server does not support
this option, it will simply ignore it.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>