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/termios_linux.go

@@ -0,0 +1,10 @@
+package mnemonic
+
+import "golang.org/x/sys/unix"
+
+// The requests that read and set the terminal settings, as
+// golang.org/x/term names them.
+const (
+	ioctlReadTermios  = unix.TCGETS
+	ioctlWriteTermios = unix.TCSETS
+)