ADVFS: Import advfs_tru64_src_v1 (GPLv2)

Import pristine open-sourced AdvFS from Tru64.

This piece of software will be ported for NetBSD,
and then requested to sublicense by vendor (HP)
to a copyleft license.
ADVFS: Add MSFS to amd64 kernel=ALL
advfs: Temporary walk-around missing /usr/include/fs/msfs