mtree(8): Replace our mtree(8) with NetBSD's version.

mtree(8): Replace our mtree(8) with NetBSD's version.

Some of its files will be used by other stuff we want to upgrade or
bring in, such as install(1) and makefs(8).

Since mtree(8) is a bootstrap tool and I want this to be painless,
I've imported some other source files that mtree(8) uses, such as
from mknod(1), which we don't have, as part of mtree(8).