bug#12985: eval-last-sexp looks broken when executed twice

[Juri Linkov]

> It seems a better design would be to always include the string, even
> the first time, when displaying in the echo area, and never include
> the string when printing into the buffer, regardless of the invoking
> command name. This is simpler, and it's how eval-expression
> already works.

This is implemented now, including other discussed changes.