Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

bsdinstall: allow an install script to access packages on the DVD

If installing from the DVD, mount its packages in the chroot at
/dist/packages. That way they'll be accessible to an install script.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: gjb
Differential Revision: https://reviews.freebsd.org/D35330

bsdinstall: don't fail if fstab is empty

PR: 210865
Submitted by: Tim Lukasiewicz <eclipzetech08@gmail.com>, h-fujishima@sakura.ad.jp (original version)
Reported by: h-fujishima@sakura.ad.jp
Reviewed by: sef
MFC after: 2 weeks
Sponsored by: Klara Inc.
Event: July 2020 Bugathon

bsdinstall: Use TMPDIR if set

Submitted by: Ryan Moeller <ryan@freqlabs.com>
Reviewed by: bcran, Nick Wolff <darkfiberiru@gmail.com>
Differential Revision: https://reviews.freebsd.org/D22979/

bsdinstall: remove EOL whitespace

Import bsdinstall. This is meant to be (eventually in conjunction with
pc-sysinstall) a replacement for sysinstall in the 9.0 release and beyond.
Currently supported platforms are sparc64, pc98, i386, amd64, powerpc, and
powerpc64. Integration into the build system will occur in the coming
weeks.

Merging with pc-sysinstall will use this code as a frontend, while
temporarily retaining the interactive partition editor here. This work
will be done in parallel with improvements on this code and release
integration.

Thanks to all who have provided testing and comments!