mk/check-wrkref: allow to disallow DESTDIR in the binary package

When a package is built, it first gets installed into DESTDIR, which is
somewhere below WRKDIR.  From there, a binary package is created, which
is then installed via pkg_add.  After this step, the value of DESTDIR is
no longer relevant, thus there is no reason to refer to that directory
from the final binary package.

This new check is not enabled by default, to allow for an opt-in testing
phase first.

(rillig)