Update fuse_kernel header.

Bring in the changes from the FUSE kernel interface 7.10
(available under a BSD license).

After 7.10 the linux FUSE developers added support for a
controversial CUSE driver and some linux especific
features that are unlikely to find its way into FreeBSD.

We currently don't implement any of the new features so we
are *not* bumping the FUSE_KERNEL_MINOR_VERSION. The header
should, nevertheless, serve  as a template to add the new
features in a compatible manner.

While here adopt some minor cleanups from the upstream version
like removing FUSE_MAJOR and FUSE_MINOR which were never
used. Also add multiple inclusion header guards,