bsdinstall netconfig: avoid duplicate entries in rc.conf

This uses sysrc to write and update configuration variables in the
temporary configuration file for network access, ._rc.conf.net. This
replaces the previous mechanism, which was simply appending new values
as they were updated.

PR: 212396
Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42194