Add new sets: base32, debug32, manhtml

- base32 contains (when MKCOMPAT=yes) shared libraries for 32-bit
compatibility, previously included in base

- debug32 contains (when MKCOMPAT=yes) debug symbols and static libraries
containing debug symbols for 32-bit compatiblity, previously included
in debug

- manhtml contains (when MKHTML=yes) the HTML files previously included
in 'man', which are of limited utility without third-party software.

The motivation for this change is to be able to easily exclude sets
from CD-ROM images that go over the size limit without xz compression
(which many NetBSD platforms struggle to extract at acceptable speeds).