Add support for backchannels to the kernel RPC. Backchannels

Add support for backchannels to the kernel RPC. Backchannels
are used by NFSv4.1 for callbacks. A backchannel is a connection
established by the client, but used for RPCs done by the server
on the client (callbacks). As a result, this patch mixes some
client side calls in the server side and vice versa. Some
definitions in the .c files were extracted out into a file called
krpc.h, so that they could be included in multiple .c files.
This code has been in projects/nfsv4.1-client for some time.
Although no one has given it a formal review, I believe kib@
has taken a look at it.