Make resizewin(1) discard the terminal queues, to lower the chance
for "unable to parse response" error which happens when youre typing
too fast for the machine you're running it on.

Reviewed by: cem, Daniel O'Connor <darius@dons.net.au>
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10624