bsdinstall: Remove unused variables in fetchmissingdists

ALL_DISTRIBUTIONS and VERIFY_MANIFEST_SIG

They are neither used in the script nor exported.
Not referenced anywhere in bsdinstall/*

Approved by: imp@
Differential Revision: https://reviews.freebsd.org/D42369

bsdinstall: avoid conflicts with fd 3

Throughout the bsdinstall script fd 3 is used by f_dprintf (set through
$TERMINAL_STDOUT_PASSTHRU). In several places in the bsdinstalls scripts,
we use fd 3 to juggle stdout when calling out to other tools, which can
cause the installer to fail with a "Bad file descriptor" error when
f_dprintf attempts to use it.

This commit replaces all constructs like this:

exec 3>&1
SOME_VARIABLE=$(some command 2>&1 1>&3)
exec 3>&-

With:

exec 5>&1
SOME_VARIABLE=$(some command 2>&1 1>&5)
exec 5>&-

PR: 273148
Reviewed by: corvink
Fixes: 1f7746d81f53447ac15cc99395bb714d4dd0a4da ("bsdinstall: stop messing with file descriptors")
MFC after: 1 week

Remove $FreeBSD$: one-line sh pattern

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

bsdinstall: use the correct DISTDIR for fetching local distfiles

fetchmissingdists naturally sets BSDINSTALL_DISTDIR to a directory in
the new filesystem that it can write fetched distfiles to. As a result,
BSDINSTALL_DISTSITE was incorrectly set to the scratch space on /mnt for
the call to distfetch when grabbing local distfiles, and it would
subsequently fail.

Switch to using the copy of BSDINSTALL_DISTDIR that we stashed off
coming into fetchmissingdists; this one is in-fact set to the path where
the local distfiles are stored.

Patch suggested by jrtc27.

Reported and tested by: Daniel O'Connor <darius dons net au>
MFC after: 1 week

bsdinstall: fix a couple stragglers in whitelabeling the scripts

PR: 265797
Reviewed by: allanjude, asiciliano
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D36235

bsdinstall: allow whitelabeling the scripts

Approved by: allanjude, asiciliano
Differential Revision: https://reviews.freebsd.org/D35197
Sponsored by: Rubicon Communications, LLC ("Netgate")

bsdinstall fetchmissingdists: Replace dialog with bsddialog

Replace (LGPL) dialog utility with (BSD-2-Clause) bsddialog utility.

Approved by: bapt (mentor)
Differential Revision: https://reviews.freebsd.org/D34654

bsdinstall: Add quotes around error message argument

When error is called with a message with spaces (and probably multiple
lines) these are passed into dialog unquoted and an error message was
presented, wrap with quotes.

Reviewed by: bapt, allanjude
Sponsored by: Ampere Computing LLC
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D33918

Fix scripted installation from media without local distfiles.

The bsdinstall script target did not have the infrastructure to fetch
distfiles from a remote server the way the interactive installer does
on e.g. bootonly media. Solve this by factoring out the parts of the
installer that deal with fetching missing distributions into a new
install stage called 'fetchmissingdists', which is called by both the
interactive and scripted installer frontends.

In the course of these changes, cleaned up a few other issues with
the fetching of missing distribution files and added a warning if
fetching the MANIFEST file, which is used to verify the integrity of
the distribution files. We should at some point add cryptographic
signatures to MANIFEST so that it can be fetched safely if not present
on the install media (which it is for bootonly media).

Initial patch by: Vinícius Zavam
PR: 255659, 250928
Reviewed by: dteske
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D27121