mk/platform/NetBSD.mk: Default to bsdtar

Native tar on NetBSD <=8 (and later, if MKBSDTAR is no) does not
handle archive formats increasingly being seen in pkgsrc.  bsdtar
("libarchive tar") does, and is natively provided on NetBSD >=9.

pkgsrc already has a mechanism to use /bin/tar as the bsdtar tool when
it is bsdtar, so this change should not cause anything different on
NetBSD >= 9.  On <=8, it will depend on archivers/bsdtar, which can
be built without using an extract tool, and which will then be broadly
usable.

As broadly discussed over multiple days on tech-pkg.

(gdt)