fix nsec's issue with file descriptors and rfork at the price of a bit of performance

Reference: /n/atom/patch/applied/nsecfdsafe
Date: Wed Feb 19 13:30:23 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/9sys/nsec.c	Wed Feb 19 13:30:18 2014
+++ /sys/src/libc/9sys/nsec.c	Wed Feb 19 13:30:18 2014
@@ -1,60 +1,18 @@
 #include <u.h>
 #include <libc.h>
-#include <tos.h>
-
-static int fd = -1;
-static struct {
-	int	pid;
-	int	fd;
-} fds[64];
 
 vlong
 nsec(void)
 {
 	uchar b[8];
-	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
-	 */
+	int fd;
 
-	/* First, look if we've opened it for this particular pid */
-	pid = _tos->pid;
-	for(tries = 0; tries < 3; tries++){
-		f = -1;
-		for(i = 0; i < nelem(fds); i++)
-			if(fds[i].pid == pid){
-				f = fds[i].fd;
-				break;
-			}
-		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 == 0){
-						fds[i].pid = pid;
-						fds[i].fd = f;
-						break;
-					}
-			}
-		}
-		if(pread(f, b, sizeof b, 0) == sizeof b)
-			return getbe(b, 8);
-		close(f);
-		if(f == fd)
-			fd = -1;
-		if(i < nelem(fds)){
-			fds[i].pid = 0;
-			fds[i].fd = -1;
-		}
+	fd = open("/dev/bintime", OREAD);
+	if(fd != -1)
+	if(pread(fd, b, sizeof b, 0) == sizeof b){
+		close(fd);
+		return getbe(b, sizeof b);
 	}
+	close(fd);
 	return 0;
 }
--- /sys/man/2/time	Wed Feb 19 13:30:19 2014
+++ /sys/man/2/time	Wed Feb 19 13:30:19 2014
@@ -64,14 +64,3 @@
 .SH DIAGNOSTICS
 Sets
 .IR errstr .
-.SH BUGS
-Both
-.I time
-and
-.I nsec
-maintain a static file descriptor.
-This can cause problems with programs
-that share memory but not file descriptors.
-Use
-.I bintime
-instead.