9p: Make transports dynamic

This patch abstracts out the interfaces to underlying transports so that
new transports can be added as modules.  This should also allow kernel
configuration of transports without ifdef-hell.

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

Documentation/filesystems/9p.txt

@@ -35,12 +35,12 @@ For remote file server:
 
 For Plan 9 From User Space applications (http://swtch.com/plan9)
 
-	mount -t 9p `namespace`/acme /mnt/9 -o proto=unix,uname=$USER
+	mount -t 9p `namespace`/acme /mnt/9 -o trans=unix,uname=$USER
 
 OPTIONS
 =======
 
-  proto=name	select an alternative transport.  Valid options are
+  trans=name	select an alternative transport.  Valid options are
   		currently:
  			unix - specifying a named pipe mount point
  			tcp  - specifying a normal TCP/IP connection
@@ -68,9 +68,9 @@ OPTIONS
 			0x40 = display transport debug
 			0x80 = display allocation debug
 
-  rfdno=n	the file descriptor for reading with proto=fd
+  rfdno=n	the file descriptor for reading with trans=fd
 
-  wfdno=n	the file descriptor for writing with proto=fd
+  wfdno=n	the file descriptor for writing with trans=fd
 
   maxdata=n	the number of bytes to use for 9p packet payload (msize)