Fix echoing of replayed keys.

Fix echoing of replayed keys.

* keyboard.c (echo_add_char): New function, factored out from
echo_char.  Don't add a space if the previous echo string was
empty.
(echo_char): Use it.
(read_key_sequence): When echoing mock input, ensure that the
trailing dash is properly added.

Fixes: debbugs:13255