freebsd-update: improve pkgbase check

The previous version used a case-insensitive match (default for -x).
The presence of packages like freebsd-git-devtools and freebsd-ftpd
would falsely trigger the packaged base check.

Instead, just use pkg which /usr/bin/uname as a packaged base
indication. pkg uses /usr/bin/uname to determine ABI, so we can rely on
it existing. If it comes from a package then packaged base is in use.

Also, extend the check to all freebsd-update commands. It is easier to
just disallow all commands, and easier to test.

Reported by: Mark Millard
Reviewed by: manu
Fixes: cf1aba2857c1 ("freebsd-update: refuse to operate on a pkgbase system")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47378