Add dtrace provider 'afp' and dtrace probes

Add dtrace provider 'afp' and dtrace probes

There are three dtrace probes:
- all AFP functions, passing the function name as argument
- AFP read and write, passing the IO size as argument
- all CNID functions

In order to be able to simply differintiate between disk IO and
network IO, replace all occurences of read and write on sockets
with send/recv, and replace the use of writev in the logging code
with two calls to write.
As a result, using dtrace probes for read() and write() syscalls
can be used to gather metrics for disk IO, while probing send(),
recv() and writev() probe network IO.