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. On the Unix systems the prompt now sets the
terminal as ReadPassword does and reads the line from standard input,
which reports the end of input; elsewhere, such as Windows, it still
uses ReadPassword. 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

@@ -136,7 +136,7 @@ func ask() (string, error) {
 
 	fmt.Fprint(os.Stderr, "mnemonic: ")
 
-	typed, err := term.ReadPassword(fd)
+	typed, err := readWithoutEcho()
 
 	fmt.Fprintln(os.Stderr)
 
@@ -144,7 +144,7 @@ func ask() (string, error) {
 		return "", fmt.Errorf("reading the mnemonic: %w", err)
 	}
 
-	return checked(string(typed))
+	return checked(typed)
 }
 
 // checked joins the words with single spaces, whatever whitespace