sysrename: fix to take full paths. Currently not quite there

This includes the changes to make sys_rename work for arbitrary paths.
There's also a bit of prototype cleanup.

Doesn't quite work yet but that may be a go9p bug. The wstat is correct
when sent now.

I'm fairly sure the chan tests are right, I talked to the guys at BL
about them.

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