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>

README.md

@@ -116,7 +116,11 @@ order; the first one found wins:
 2. Environment variable `KEYFUNC_MNEMONIC_COMMAND`: the same, as a shell command
    held in the environment.
 3. Environment variable `KEYFUNC_MNEMONIC`: the mnemonic itself.
-4. A prompt on the terminal with echo turned off.
+4. A prompt on the terminal with echo turned off. The mnemonic is what is typed
+   up to Enter, and echo is turned back on once it is read. Ctrl-D with nothing
+   typed ends the prompt at once and leaves the mnemonic empty, so the tool
+   refuses it and exits with status 1. After text has been typed, Ctrl-D has to
+   be pressed twice to end the prompt, and the text typed is the mnemonic.
 
 If none of these is available and standard input is not a terminal, the tool
 refuses and exits with status 1. A mnemonic that is empty, does not have 12, 15,