loader's GPT support on BIOS does not seem to like the root filesystem
being the last filesystem on the disk for some reason when made by this
script. Add a vestigial swap partition to allow this to boot with QEMU
BIOS.
Turn off the full witness trace on console output.

On head/, or more specifically, when WITNESS is in
the kernel config, the console is spammed excessively
with lock order reversal between isofs and devfs.

Set debug.witness.trace=0 in the installer sysctl.conf
to avoid printing the full KDB stack backtrace.  This
does not prevent printing the lock order reversal has
happened, only lessens the console spam.

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