Fix a case problem on a struct name

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Add the mount device.

This required a bit more code, we needed the conv funtions for 9p packets,
some changes to fcall.h and plan9.h, the device itself (mnt.c) and some
corrections to spatch scripts.

We're getting dangerously close to having picked the bits out of NIX that we wanted.

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