Add additional functionality to the syslink implementation. Give the

Add additional functionality to the syslink implementation.  Give the
kernel side the ability to send a syslink message asynchronously (we we
can implement an asynchronous VOP_STRATEGY) and add support code to
construct XIO's for kernel consumption.

Document the syslink element flags a bit better and add a ne wsyslink
element construction inline which constructs a simple degenerate element.