Add command line argument to disable DHCP options parsing

Add command line argument to disable DHCP options parsing

Default routes added when parsing DHCP options are problematic on
setups where there are more interfaces than those being managed by
relayd. A default route on the routing table used for locally
generated traffic makes the traffic addressed to not managed local
networks to be sent out using that default route instead of being
properly routed.

Disabling DHCP options parsing prevents the introduction of a default
route, that way the traffic addressed to not managed local networks is
routed using the main routing table.

Signed-off-by: Alejandro Enrique <alejandro.enrique@fon.com>