Avoid kernels between r224778 and r224841; bug causes buildworld failure

Dear all:

As you may have seen from current_at_ traffic, a bug crept in during the Capsicum 
merge, introduced by the infamous Last Minute Cleanup and not caught in 
pre-commit testing.  The most noticed effect of the bug is to cause buildworld 
to fail due to a problem with /dev/{stdin,stdout,stderr} when running an 
affected kernel.  The bug doesn't break buildkernel, so it's possible to 
update from bad revisions by doing that (or rolling back to a pre-update 
kernel).  The problem appeared in r224778 (11 August), and was fixed in 
r224842 (13 August), so it's a pretty short window and hopefully the problem 
will scroll out quickly.

I'm currently awaiting approvel from re_at_ to commit a note to UPDATING saying 
the above.

Sorry about that!

Robert N M Watson
Computer Laboratory
University of Cambridge