Implement the nanotime system call.

This is notably useful for the port of the Go Programming Language
on Plan 9. In the Go environment, we are currently relying on the
/dev/bintime file to get time. However it has the disadvantage of
introducing latency (in order of ten microseconds) and require the
/dev/bintime file to be opened for every child process and call
to the nanotime() function.

The nanotime syscall should also be implemented in /sys/src/9k/port/sysfile.c:

void
sysnanotime(Ar0* ar0, va_list)
{
	/*
	 * vlong nanotime(void);
	 */
	ar0->vl = todget(nil);
}

This is an idea from Ron Minnich, which was originaly implemented
as part of NxM. I think Jim already reviewed this change last year.

Reference: /n/sources/patch/maybe/sys-nanotime
Date: Wed Jan 15 20:25:36 CET 2014
Signed-off-by: djc@9grid.fr

--- /sys/src/libc/9syscall/sys.h	Wed Jan 15 20:24:35 2014
+++ /sys/src/libc/9syscall/sys.h	Wed Jan 15 20:24:32 2014
@@ -49,3 +49,4 @@
 #define	PREAD		50
 #define	PWRITE		51
 #define	TSEMACQUIRE	52
+#define	NANOTIME	53
--- /sys/src/9/port/sysfile.c	Wed Jan 15 20:24:50 2014
+++ /sys/src/9/port/sysfile.c	Wed Jan 15 20:24:42 2014
@@ -281,6 +281,14 @@
 	return fd;
 }
 
+long
+sysnanotime(ulong *arg)
+{
+	validaddr(arg[0], BY2V, 1);
+	*(vlong*)arg[0] = todget(nil);
+	return 0;
+}
+
 void
 fdclose(int fd, int flag)
 {
--- /sys/include/libc.h	Wed Jan 15 20:25:04 2014
+++ /sys/include/libc.h	Wed Jan 15 20:24:55 2014
@@ -668,6 +668,7 @@
 extern	int	mount(int, int, char*, int, char*);
 extern	int	unmount(char*, char*);
 extern	int	noted(int);
+extern	vlong	nanotime(void);
 extern	int	notify(void(*)(void*, char*));
 extern	int	open(char*, int);
 extern	int	fd2path(int, char*, int);
--- /sys/src/libc/9sys/nsec.c	Wed Jan 15 20:25:09 2014
+++ /sys/src/libc/9sys/nsec.c	Wed Jan 15 20:25:06 2014
@@ -1,76 +1,8 @@
 #include <u.h>
 #include <libc.h>
-#include <tos.h>
-
-static uvlong order = 0x0001020304050607ULL;
-
-static void
-be2vlong(vlong *to, uchar *f)
-{
-	uchar *t, *o;
-	int i;
-
-	t = (uchar*)to;
-	o = (uchar*)&order;
-	for(i = 0; i < sizeof order; i++)
-		t[o[i]] = f[i];
-}
-
-static int fd = -1;
-static struct {
-	int	pid;
-	int	fd;
-} fds[64];
 
 vlong
 nsec(void)
 {
-	uchar b[8];
-	vlong t;
-	int pid, i, f, tries;
-
-	/*
-	 * Threaded programs may have multiple procs
-	 * with different fd tables, so we may need to open
-	 * /dev/bintime on a per-pid basis
-	 */
-
-	/* First, look if we've opened it for this particular pid */
-	if((pid = _tos->pid) == 0)		/* 9vx bug, perhaps? */
-		_tos->pid = pid = getpid();
-	do{
-		f = -1;
-		for(i = 0; i < nelem(fds); i++)
-			if(fds[i].pid == pid){
-				f = fds[i].fd;
-				break;
-			}
-		tries = 0;
-		if(f < 0){
-			/* If it's not open for this pid, try the global pid */
-			if(fd >= 0)
-				f = fd;
-			else{
-				/* must open */
-				if((f = open("/dev/bintime", OREAD|OCEXEC)) < 0)
-					return 0;
-				fd = f;
-				for(i = 0; i < nelem(fds); i++)
-					if(fds[i].pid == pid || fds[i].pid == 0){
-						fds[i].pid = pid;
-						fds[i].fd = f;
-						break;
-					}
-			}
-		}
-		if(pread(f, b, sizeof b, 0) == sizeof b){
-			be2vlong(&t, b);
-			return t;
-		}
-		close(f);
-		if(i < nelem(fds))
-			fds[i].fd = -1;
-	}while(tries++ == 0);	/* retry once */
-	USED(tries);
-	return 0;
+	return nanotime();
 }