Fix scripted installs on EFI systems after default mounting of the ESP.

Because the ESP mount point (/boot/efi) is in mtree, tar will attempt to
extract a directory at that point post-mount when the system is installed.
Normally, this is fine, since tar can happily set whatever properties it
wants. For FAT32 file systems, however, like the ESP, tar will attempt to
set mtime on the root directory, which FAT does not support, and tar will
interpret this as a fatal error, breaking the install (see
https://github.com/libarchive/libarchive/issues/1516). This issue would
also break scripted installs on bare-metal POWER8, POWER9, and PS3
systems, as well as some ARM systems.

This patch solves the problem in two ways:

• If stdout is a TTY, use the distextract stage instead of tar, as in interactive installs. distextract solves this problem internally and provides a nicer UI to boot, but requires a TTY.
• If stdout is not a TTY, use tar but, as a stopgap for 13.0, exclude boot/efi from tarball extraction and then add it by hand. This is a hack, and better solutions (as in the libarchive ticket above) will obsolete it, but it solves the most common case, leaving only unattended TTY-less installs on a few tier-2 platforms broken.

In addition, fix a bug with fstab generation uncovered once the tar issue
is fixed that umount(8) can depend on the ordering of lines in fstab in a
way that mount(8) does not. The partition editor now writes out fstab in
mount order, making sure umount (run at the end of scripted, but not
interactive, installs) succeeds.

PR: 254395
Approved by: re (gjb)
Reviewed by: gjb, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29380

(cherry picked from commit c2f16c595eb51c6e0cb6ece3f6f078d738019059)
(cherry picked from commit 4601382e1362352f17a33e4ed38db5dcfe3f6be5)