MFC r332390: make-memstick.sh: put partition args on separate lines

This makes it easier to identify the individual partition types and
facilitates comparisons across architectures.

Sponsored by: The FreeBSD Foundation

MFC r323438: make-memstick.sh: use UFSv2

There's not much practical difference as far as install media is
concerned but newfs creates UFSv2 by default and it is sensible to use
the contemporary UFS version.

I also intend to change makefs to create UFSv2 by default (to match
newfs) so we'll want make-memstick.sh to be explicit, rather than
relying on the host tool's default.

Sponsored by: The FreeBSD Foundation

make-memstick.sh: use 'set -e' to abort if any step fails

Also remove the now-redundant error handling that was only for makefs.

This script was run on an older FreeBSD host that lacked efi-on-mbr
support in makefs. A warning was emitted on the console (from makefs)
but the script continued running and exited with 0.

MFC of r308171 (arm64) and r323178 (other architectures).

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Prevent memstick installation medium from attempting to mount
the root filesystem read-write. This causes problems booting
the memstick installation medium from write-protected USB flash
drives.

Submitted by: A.J. Kehoe IV [1], Oliver Jones [2]
PR: 187161 [1], 205886 [2]
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

Use mkimg instead of md(4) and gpart.

Mount memsticks read-only by default to prevent them being filled by
user modifications and subsequently preventing a functioning installation.

Approved by: re (kib)

Add support for memstick generation on PowerPC. This is a little suboptimal
since glabel doesn't know about APM partitioning yet, but works well enough.