kernel&all: create() syscall only sends Tcreate

In Plan9 the create syscall fallback on a open(OTRUNC) if the
path provided already exists. This is actually a common requirement
as most programs (editors, cat...) simply requires that a file is
there and is empty, and doesn't care overwriting existing contents
(note that this is particularily sensible with something like fossil).

In Jehanne the application is responsible of actually handle this
"file exists" error but libc provides ocreate() to mimic the Plan9
behaviour. Note that ocreate introduce a subtle race too: the path
is walked several times if the file exists, thus it could misbehave
on concurrent namespace changes. However I guess this is not going to
happen often enough to care now.

NOTE we will probably address this rare race too, with a more drammatic change
to syscalls: a new walk() syscall that will provide an unopen fd.

sys/include/9P2000.h

@@ -20,10 +20,11 @@
 #define	NP_ORDWR	2	/* read and write */
 #define	NP_OEXEC	3	/* execute, == read but check execute permission */
 #define	NP_OTRUNC	16	/* or'ed in (except for exec), truncate file first */
+#define NP_OCEXEC	32	/* or'ed in, close on exec */
 #define	NP_ORCLOSE	64	/* or'ed in, remove on close */
 
 /* bits that must be zero in open/create mode */
-#define NP_OZEROES	~(NP_OREAD|NP_OWRITE|NP_ORDWR|NP_OEXEC|NP_OTRUNC|NP_ORCLOSE)
+#define NP_OZEROES	~(NP_OREAD|NP_OWRITE|NP_ORDWR|NP_OEXEC|NP_OTRUNC|NP_OCEXEC|NP_ORCLOSE)
 
 typedef enum NinepMsgType
 {