Render UI directly to /dev/tty

See https://github.com/junegunn/fzf/discussions/3792

This allows us to separately capture the standard error from fzf and its
child processes, and there's less chance of user errors of redirecting
the error stream and hiding fzf.

src/tui/light_unix.go

@@ -14,6 +14,8 @@ import (
 	"golang.org/x/term"
 )
 
+var tty string
+
 func IsLightRendererSupported() bool {
 	return true
 }
@@ -48,12 +50,14 @@ func (r *LightRenderer) closePlatform() {
 	// NOOP
 }
 
-func openTtyIn() (*os.File, error) {
-	in, err := os.OpenFile(consoleDevice, syscall.O_RDONLY, 0)
+func openTty(mode int) (*os.File, error) {
+	in, err := os.OpenFile(consoleDevice, mode, 0)
 	if err != nil {
-		tty := ttyname()
+		if len(tty) == 0 {
+			tty = ttyname()
+		}
 		if len(tty) > 0 {
-			if in, err := os.OpenFile(tty, syscall.O_RDONLY, 0); err == nil {
+			if in, err := os.OpenFile(tty, mode, 0); err == nil {
 				return in, nil
 			}
 		}
@@ -62,6 +66,14 @@ func openTtyIn() (*os.File, error) {
 	return in, nil
 }
 
+func openTtyIn() (*os.File, error) {
+	return openTty(syscall.O_RDONLY)
+}
+
+func openTtyOut() (*os.File, error) {
+	return openTty(syscall.O_WRONLY)
+}
+
 func (r *LightRenderer) setupTerminal() {
 	term.MakeRaw(r.fd())
 }