Use `auth-source-read-char-choice' instead of `read-char'.

Use `auth-source-read-char-choice' instead of `read-char'.

* auth-source.el (auth-source-read-char-choice): New function to read a
character choice using `dropdown-list', `read-char-choice', or
`read-char'.  It appends "[a/b/c] " to the prompt if the choices were
'(?a ?b ?c).  The `dropdown-list' support is disabled for now.
(auth-source-netrc-saver): Use it.