[PATCH] 9p: update documentation regarding server applications

Update the documentation to cover using Inferno as a server for 9p and to
include information about spfs (a stable single-threaded stand-alone 9p
server).

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>