freeze for sbcl-2.6.1

Hi,

Well, it's been an eventful month, at least from the "platform support"
side in upcoming NEWS entries.  In order to not have to deal with too
much, please let's stop new feature work and instead deal with any
issues that might have arisen over the course of the last few weeks,
concentrating on making sure that any changes are of the clearly benign
kind.

Thanks,

Christophe