sysinst disallows FFSv2 root (ofppc, evbppc)

Hi,

I wanted to rotate the flag do decide whether FFSv2 is allowed as root,
thinking most cases are wrong. that turned out to be false, so I'll
note: it is probably wrong for evbppc and ofppc, which currently don't
claim to support it. It may be desirable for some.

I understand the main points of FFSv2 are:
- lazy inode initialization as opposed to at newfs time, which is better
  for larger disks.
- inlined symlinks, below some length ("maxsymlinklen")
- on-disk time format is 64bit, but we will probably implement epoch
  rollover for FFSv1 anyway, as some archs require it like shark whose
  native bootloader(!) does FFSv1
- ?? more things, I'm not deeply aware of the differences.

Downsides:
- It appears that bootloaders are bigger if they support it, so weaker
  archs don't support it.