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
types Ctrl-D at the prompt on a pseudo-terminal.

Unverified: the prompt was run only on Linux; other systems were built.
Deviation: a Ctrl-H byte is no longer a backspace at the prompt.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/mnemonic/prompt_notunix.go

@@ -0,0 +1,18 @@
+//go:build !aix && !darwin && !dragonfly && !freebsd && !linux && !netbsd && !openbsd && !solaris && !zos
+
+package mnemonic
+
+import (
+	"os"
+
+	"golang.org/x/term"
+)
+
+// readWithoutEcho reads one line from the terminal on standard input
+// with echo turned off, through term.ReadPassword, on the systems
+// prompt_unix.go is not built for, such as Windows.
+func readWithoutEcho() (string, error) {
+	typed, err := term.ReadPassword(int(os.Stdin.Fd()))
+
+	return string(typed), err
+}