r3039: This solves the problem of async handlers in ntvfs backends not being

r3039: This solves the problem of async handlers in ntvfs backends not being
in the right state when called. For example, when we use the unixuid
handler in the chain of handlers, and a backend decides to continue a
call asynchronously then we need to ensure that the continuation
happens with the right security context.

The solution is to add a new ntvfs operation ntvfs_async_setup(),
which calls all the way down through the layers, setting up anything
that is required, and takes a private pointer. The backend wanting to
make a async calls can use ntvfs_async_setup() to ensure that the
modules above it are called when doing async processing.
(This used to be commit a256e71029727fa1659ade6257085df537308c7d)