Add define to enable autotrim to default to on

In FreeBSD trim has defaulted to on for several
years. In order to minimize POLA violations on
import it's important to maintain this default
when importing vendored openzfs in to FreeBSD
base.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10719