Write works.

I'm going to yank a lot of these debug prints at some point but
they're handy.

Turns out write always worked, I was opening the file readonly and the
kernel objected.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Stitch plan 9 open into sys_open

This seems to be working; the oflags are bog-standard unix. Not sure
how to manage the omode yet.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>