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/devcons.c

@@ -787,7 +787,7 @@ consstat(Chan *c, uint8_t *dp, long n)
 }
 
 static Chan*
-consopen(Chan *c, int omode)
+consopen(Chan *c, unsigned long omode)
 {
 	c->aux = nil;
 	c = devopen(c, omode, consdir, nelem(consdir), devgen);