9p + ix = 9pix

More experimentation with ix leaded to attempts to put it in a plan 9 kernel.
To make a start, we changed fossil to accept groups of requests.
A group is just a series of requests sharing the same tag, like in ix.

A new mount driver is on the works, along with experimental interfaces for
the rest of the kernel to issue calls.

One result is that it seems easier if the kernel forwards Call structures
instead of using procedural interfaces.

Another is that it seems that to exploit streaming and concurrency the calls
require using futures or promises. For example, a write call returns a
handle that can be used later to check out the results from the write.

Using this it is easier to, for example, issue all the clone requests at once
when forking a name space, and then collect all the replies. Or, to issue
a sequence of walk, open, read requests to pull data into a cache.

The plan is to make this variant, called 9pix, evolve up to a compromise
point where we get the best of ix while we keep as compatible with 9p as
possible.

More on this when we actually get the toy working...