decouple Jehanne from 9P2000 (as much as needed)

Jehanne is going to use a new file protocol, but Plan 9 is really
coupled with 9P2000.

Renamed fcall.h as 9P2000.h and introduced specific constants such
as NP_OREAD, NP_OWRITE and so on, so that we can use different values
in the kernel and new protocol.

Renamed devmnt to devninep, since it's actually a device serving 9P2000
file systems.

Also, fixed 9P2000 support in Jehanne, that was broken with the introduction
of OSTAT.

sys/src/kern/port/devpipe.c

@@ -171,7 +171,7 @@ pipestat(Chan *c, uint8_t *db, long n)
  *  if the stream doesn't exist, create it
  */
 static Chan*
-pipeopen(Chan *c, int omode)
+pipeopen(Chan *c, unsigned long omode)
 {
 	Pipe *p;