[9fans] mount 9P on Linux and FreeBSD via FUSE

["Russ Cox" <rsc@swtch.com>]

Various changes to p9p over the weekend;
see http://swtch.com/usr/local/plan9/CHANGES.

The changes center around a new program called 9pfuse
(inspired by Christoph Lohmann's program of the same name)
that mounts 9P servers on Linux and FreeBSD via FUSE.
9pfuse uses the raw FUSE message format instead of using
the high-level library interface, which actually hides some
details important to a faithful translation.

The net result is that I can do:

	x40=; 9 mount `namespace`/acme /mnt/acme
	x40=; ls /mnt/acme
	309  355  356  acme  cons  consctl  draw  editout  index  label  new
	x40=; cat /mnt/acme/356/body
	Hello, FUSE!
	x40=; unmount /mnt/acme
	x40=;

Thanks to Lou Kamenov for working through some FreeBSD
bugs.  If you want to use 9pfuse on FreeBSD you'll need the
very latest FUSE implementation (this morning's), which
fixes some protocol errors on the part of the kernel module.

And thanks to Christoph Lohmann for writing the other 9pfuse.

Russ