bug#15568: Lookup this-command-keys without a prefix argument

[Juri Linkov]

>> As discovered in bug#15200, (lookup-key isearch-mode-map (this-command-keys))
>> can't decide whether the key sequence is bound to an isearch command or not.
>
> Actually, this is easy to fix: use this-single-command-keys.

Ah, I expected such a command, but missed it when looked
at the list of commands with the prefix "this-command-...".

Also I discovered that to support a key sequence like
`C-s C-u C-u C-l' requires adding the command `universal-argument-more'
(and `universal-argument-minus' for completeness).

I installed a fix for these problems.