syscall/exec_linux: enable changing controlling tty

As discussed in the following thread:
https://groups.google.com/forum/?fromgroups=#!topic/golang-dev/emeJffxWhVo

This is required to enable applications such as `less` to use something
other than stdin as the controlling terminal.

R=dave, iant
CC=bradfitz, golang-dev
https://golang.org/cl/6785057

src/pkg/syscall/exec_linux.go

@@ -16,8 +16,9 @@ type SysProcAttr struct {
 	Ptrace     bool        // Enable tracing.
 	Setsid     bool        // Create session.
 	Setpgid    bool        // Set process group ID to new pid (SYSV setpgrp)
-	Setctty    bool        // Set controlling terminal to fd 0
+	Setctty    bool        // Set controlling terminal to fd Ctty (only meaningful if Setsid is set)
 	Noctty     bool        // Detach fd 0 from controlling terminal
+	Ctty       int         // Controlling TTY fd (Linux only)
 	Pdeathsig  Signal      // Signal that the process will get when its parent dies (Linux only)
 }
 
@@ -206,9 +207,9 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		}
 	}
 
-	// Make fd 0 the tty
-	if sys.Setctty {
-		_, _, err1 = RawSyscall(SYS_IOCTL, 0, uintptr(TIOCSCTTY), 0)
+	// Set the controlling TTY to Ctty
+	if sys.Setctty && sys.Ctty >= 0 {
+		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
 		if err1 != 0 {
 			goto childerror
 		}