EC2: Set dhclient_arpwait="NO"

The DHCP server in EC2 knows exactly which system should be using
which IP address (and in fact EC2 has source IP filtering on by
default) so there's no point ARPing an address before using it.

The preceding commits (changing the ARP wait time from 2 s to 250 ms)
and this one (eliminating the wait entirely in EC2) reduce the time
required for a newly launched FreeBSD/EC2 instance to launch by 2
seconds.

Discussed with: icattard
MFC after: 10 days
Sponsored by: Amazon