HEADS UP: install new ld.elf_so!

I just fixed a few bugs in ld.elf_so that prevented me from commiting
some code from Chuck to finally enable topdown-vm. I plan to do that
in about two weeks.

This means: if you "recently" updated your userland -- easy check:

 > file /bin/ls 

mentions a compiler model "medmid", like:

  /bin/ls: ELF 64-bit MSB executable, SPARC V9, relaxed memory ordering, version 1 (SYSV), dynamically linked (uses shared libs), compiler model: medmid, for NetBSD 6.99.37, not stripped

you HAVE TO update /libexec/ld.elf_so to a version build from -current
sources BEFORE you boot a kernel with topdown VM enabled.

If you fail to do this, the machine will not be able to exec init - however,
recovery is simple: boot -a and use /rescue/init and /rescue/sh, then manually
copy over a newer ld.elf_so.

If your userland is older (i.e. the above file(1) test fails), you do not need
to worry - as usual, binary compatibility is provided.

Sorry for not catching these issues earlier.
I will give another heads up when topdown has been commited.

Martin