MFC r266107,r266108,r266109:

Here is a patch for the bsdinstall root-on-zfs stuff that adds optional
encryption for swap, and optional gmirror for swap (which can be combined)

Updates to the datasets created by zfsboot.

Set compress=lz4 for the entire pool, removing it from the individual
datasets

Remove exec=no from /usr/src, breaks the test suite.

Fix the "disks" variable reuse.

It starts off being used to track the grammar for the number of disks
(singular vs plural) and then it is reused as the list of available disks.

Replace the variable with disks_grammar and move 'disk' and 'disks' to
msg_ vars so they can be translated in the future.

Submitted by:	Allan Jude <freebsd@allanjude.com>
Reviewed by:	roberto
Sponsored by:	ScaleEngine Inc.
MFC r266780:
Remove the assert which can be triggered by the userspace.