mtree: Add package tags to some root directories

This ensures the normally-empty directories /proc, /net, /media and /mnt
are created on a fresh pkgbase install.

Put /proc, /mnt and /media in -runtime since those are basic directories
that should always exist. /net is only used by the automounter, so put
that in -autofs. While here, also tag /rescue in -rescue, since nothing
else installs there.

After discussion with cperciva, this is being MFC'd early so we can land
it in the 15.0-ALPHA3 build.

Reported by: Graham Perrin <grahamperrin@gmail.com>
PR: 289654
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D52622

(cherry picked from commit b3468202994f6856c5c49f3365e05f4f8017d680)