the terminal work properly out of the box when logging over a serial
line, which is quite important for the user experience on boards like
Raspberry Pi. It doesn't affect cases where the terminal size is
already non-zero, such as SSH or vt(4) sessions.
Note that this doesn't handle a scenario pointed out by rgrimes@:
when the terminal is resized after login, the terminal size won't
get updated even after logging out and back in.
Reviewed by: imp
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10642
dd0e1324ba