Many programs use this functionality so it should be available centrally
rather than compiled into each program independently. This has the
modest downside of adding libutil dependencies to some mount_<foo>
programs, but many (plus mount(8)) already have those depndencies so
this shouldn't be a major imposition.
In commit 906c312bbf74, Kirk said this could be done once the change was
merged to FreeBSD 13. That happened in commit 668dfa016698 in January
2023.