9p: move dirread to fs layer

Currently reading a directory is implemented in the client code.
This function is not actually a wire operation, but a meta operation
which calls read operations and processes the results.

This patch moves this functionality to the fs layer and calls component
wire operations instead of constructing their packets.  This provides a
cleaner separation and will help when we reorganize the client functions
and protocol processing methods.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>