[9fans] plan9port update

[Russ Cox <russcox@gmail.com>]

There is a big update to Plan 9 port available now, at
http://swtch.com/plan9port.

The most significant user-visible change is that the entire system is
documented.  Intro(1) is a good starting point.

The most significant code change is that the thread library now uses
pthreads and ucontext(3) when possible.  This means that programs should
work properly on Linux 2.6 now, and that real threading can be used on
SunOS and Mac OS X now.  It also means porting the system is easier --
there's no sparc assembly at all in the tree, for example.  Finally, it means
that various hangs people saw when using the acme Edit command
are finally gone.

There are many many small improvements since the last real release,
far too many to list individually.   Intro(1) gives a list of man pages that
describe interesting differences from Plan 9 proper.  

Thanks to everyone who helped test for the last week or two.
I've built and run the system on Mac OS X 10.3, Linux 2.4, 2.6, FreeBSD 5,
and SunOS 5.9 today, but there may well be small things that need to be
fixed still for other platforms.  If you're feeling a bit conservative you
might wait a week or two to let others find those.

Enjoy.
Russ