syscall: allow processes steal a controlling terminal on Linux

The current implementation of forkAndExecInChild for Linux does not allow
spawned processes steal the controlling terminal from a different session
group. This patch passes 1 as the argument to TIOCSCTTY in order to allow
spawned processes steal controlling terminals.

Fixes #20454

Change-Id: I171b8981509d648b07f89bddc1e9d45cb70e00e6
Reviewed-on: https://go-review.googlesource.com/44343
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: Julio Montes

src/syscall/exec_linux.go

@@ -342,7 +342,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 
 	// Set the controlling TTY to Ctty
 	if sys.Setctty {
-		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 0)
+		_, _, err1 = RawSyscall(SYS_IOCTL, uintptr(sys.Ctty), uintptr(TIOCSCTTY), 1)
 		if err1 != 0 {
 			goto childerror
 		}