makefs/zfs: Allow the autotrim pool property to be set

[rcm]

Summary

Add an "autotrim" option taking "on" or "off". The value is stored in
the pool properties object alongside "bootfs", so the imported pool
starts trimming freed space automatically without a separate
"zpool set" step. When the option is omitted nothing is written and
the pool keeps the default ("off"), exactly as before.

A string option is used rather than makefs's boolean option type since
the latter ignores its value, which would make "autotrim=off" enable
the feature.

Define the spa_autotrim_t enumeration in the boot zfsimpl.h, copied
from OpenZFS's sys/spa.h, so that makefs can refer to SPA_AUTOTRIM_OFF
and SPA_AUTOTRIM_ON by name. The loader does not use it.

Document the option in makefs(8) and add a regression test.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

Test Plan

Regression test included

[markj]

Looks fine to me aside from a couple of nits. Please go ahead and commit without re-uploading if you want.

sys/cddl/boot/zfs/zfsimpl.h

913

This isn't really the right place to declare what the default is. That's up to the consumer, this header isn't specific to makefs.

usr.sbin/makefs/zfs.c

260

Could you please add an explicit initialization zfs->autotrimval = SPA_AUTOTRIM_OFF for the default case? I know it's redundant, but having it spelled out is a bit nicer for readers.