Hi folks...

I've just committed a simple but rather large set of changes that
make the system a lot more robust in extremely low memory conditions.
That is, it is now possible for pmap_enter() to return failure-due-to-
resource-shortage to the upper layers, so that the upper layers can
gracefully deal with the shortage.

The pmap has to support this; so far, only the Alpha port's pmap has been
updated to take advantage of this new feature.  Other systems should work
as before.

If you encounter any problems, or spot a pmap_enter() call that I forgot
to update, please send a bug report w/ send-pr(1) and I'll deal with it
as quickly as possible.

You'll also note that I bumped the kernel version for NetBSD 1.4N.  We're
running out of letters!  :-)

Have fun!

        -- Jason R. Thorpe <thorpej@nas.nasa.gov>