Don't allow unmapped I/O. The pmap isn't quite up to the task. Add a
comment to this effect and switch the default. My old AT91SAM9G20
now boots, fsck's the SD card and runs w/o an issue for the first
time since a 9.1-ish stable build I did a few years ago.

Problems with unmapped I/O:
o un-page-aligned I/O requests to devices fail (notably fsck

and newfs).

o write-back caching was totally broken. write-through caching

needed to be enabled.

o Even page-aligned I/O requests sometimes failed for reasons

not thoroughly investigated.

Suggested by: ian@
MFC after: 2 days