Add resizewin -z. It makes resizewin not do anything if the terminal
size is already set to something other than zero. It's supposed to be
called from eg /etc/profile - it's not neccessary to query terminal
size when logging in over the network, because the protocol used already
takes care of this, but it's neccessary when logging over a serial line.

Reviewed by: cem, Daniel O'Connor <darius@dons.net.au>
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10637