gvinum: Emit deprecation notice upon drive tasting

Reviewed by: phk, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38607

(cherry picked from commit 127b443124d1e720b246ad381648c436c3d240de)
(cherry picked from commit 22d06df419e0fdf7670e96d4ba88e33933cab6da)