syscall: fix duplicate fd bug for Plan 9

This change comes from CL 5536043,
created by Andrey Mirtchovski. His
description follows:

"The plan9 exec child handler does not manage
dup-ed fds from the parent correctly: when a
dup-ed file descriptor appears in the child's fd
list it is closed when first encountered and then
subsequent attempt to dup it later in Pass 2 fails,
resulting in 'fork/exec: fd out of range or not
open'."

R=golang-dev, rminnich, ality
CC=golang-dev, mirtchovski, rsc
https://golang.org/cl/6009046

src/pkg/syscall/exec_plan9.go

@@ -287,7 +287,13 @@ func forkAndExecInChild(argv0 *byte, argv []*byte, envv []envItem, dir *byte, at
 		if int(r1) == -1 {
 			goto childerror
 		}
-		RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
+	}
+
+	// Pass 3: close fds that were dup-ed
+	for i = 0; i < len(fd); i++ {
+		if fd[i] >= 0 && fd[i] != int(i) {
+			RawSyscall(SYS_CLOSE, uintptr(fd[i]), 0, 0)
+		}
 	}
 
 	// Time to exec.