End the mnemonic prompt at end of input (closes #78)

term.ReadPassword reads the terminal with a raw read that returns 0
bytes at the end of input and takes that as nothing yet, so Ctrl-D at
the prompt did nothing. The prompt now sets the terminal as
ReadPassword does and reads the line from standard input, which
reports the end of input. Ctrl-D with nothing typed gives an empty
mnemonic, refused with status 1, and the terminal settings are put
back.

A test runs the tool on a pseudo-terminal (github.com/creack/pty, at a
commit past v1.1.24, whose Open on Linux can return another pty's
terminal) and types Ctrl-D at the prompt.

Model: opus-5-5

internal/mnemonic/mnemonic.go

@@ -2,15 +2,18 @@
 package mnemonic
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"slices"
 	"strings"
 
+	"golang.org/x/sys/unix"
 	"golang.org/x/term"
 	"sneak.berlin/go/keyfunc/internal/bip39"
 )
@@ -126,7 +129,10 @@ func ChildEnv() []string {
 	return kept
 }
 
-// ask prompts on the terminal with echo turned off.
+// ask prompts on the terminal with echo turned off and reads one line,
+// which ends at a newline or at the end of input (Ctrl-D). The terminal
+// is set as term.ReadPassword sets it; ReadPassword itself is not used
+// because it never sees the end of input and goes on waiting.
 func ask() (string, error) {
 	fd := int(os.Stdin.Fd())
 
@@ -134,17 +140,36 @@ func ask() (string, error) {
 		return "", ErrMissing
 	}
 
+	settings, err := unix.IoctlGetTermios(fd, ioctlReadTermios)
+	if err != nil {
+		return "", fmt.Errorf("reading the terminal settings: %w", err)
+	}
+
+	hidden := *settings
+	hidden.Lflag &^= unix.ECHO
+	hidden.Lflag |= unix.ICANON | unix.ISIG
+	hidden.Iflag |= unix.ICRNL
+
+	err = unix.IoctlSetTermios(fd, ioctlWriteTermios, &hidden)
+	if err != nil {
+		return "", fmt.Errorf("turning echo off: %w", err)
+	}
+
+	defer func() { _ = unix.IoctlSetTermios(fd, ioctlWriteTermios, settings) }()
+
 	fmt.Fprint(os.Stderr, "mnemonic: ")
 
-	typed, err := term.ReadPassword(fd)
+	// With ICANON set, a read from the terminal returns at most one
+	// line, so the buffer takes nothing past the newline.
+	typed, err := bufio.NewReader(os.Stdin).ReadString('\n')
 
 	fmt.Fprintln(os.Stderr)
 
-	if err != nil {
+	if err != nil && !errors.Is(err, io.EOF) {
 		return "", fmt.Errorf("reading the mnemonic: %w", err)
 	}
 
-	return checked(string(typed))
+	return checked(typed)
 }
 
 // checked joins the words with single spaces, whatever whitespace