release: Use full window size for installer over serial lines

When running over a serial line we end up defaulting to 80x24, which is
rather cramped for many dialog boxes and occupies very little screen
space for most modern terminals. Thus, run resizewin -z to set the
terminal size if not already known before starting the installer, just
as we do for csh and sh login shells already in their default dotfiles.

Reviewed by: jhb, gjb
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D34414