script: handle terminal resize on SIGWINCH

Add a -w flag to forward terminal resize events on to the child, which
can be useful in some circumstances to avoid terminal corruption.

Reviewed by: des
Co-authored-by: Xavier Beaudouin <xavier.beaudouin@klarasystems.com>
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D44167