Use golang.org/x/sys/unix for IsTerminal on *BSD

Use unix.IoctlGetTermios to implement IsTerminal on *BSD and no longer
requires to use the frozen syscall package.

isatty_bsd.go

@@ -3,18 +3,12 @@
 
 package isatty
 
-import (
-	"syscall"
-	"unsafe"
-)
-
-const ioctlReadTermios = syscall.TIOCGETA
+import "golang.org/x/sys/unix"
 
 // IsTerminal return true if the file descriptor is terminal.
 func IsTerminal(fd uintptr) bool {
-	var termios syscall.Termios
-	_, _, err := syscall.Syscall6(syscall.SYS_IOCTL, fd, ioctlReadTermios, uintptr(unsafe.Pointer(&termios)), 0, 0, 0)
-	return err == 0
+	_, err := unix.IoctlGetTermios(int(fd), unix.TIOCGETA)
+	return err == nil
 }
 
 // IsCygwinTerminal return true if the file descriptor is a cygwin or msys2