bug#13480: 24.3.50; `C-w' from Isearch should translate newlines to spaces

[Juri Linkov]

>>> So we could use `isearch-message' to display the translated
>>> search string but not to translate it in `isearch-string'.
>> Yes, +1.
>
> IIRC there's the same problem with case-sensitivity.

Not downcasing the yanked string could help to fix bug#10118.

According to the docstring of `search-upper-case',
if its value is `not-yanks', text yanked into the
search string is always downcased.

So maybe a better default value for `search-upper-case'
would be `nil' instead of the current default `not-yanks'.

If a tri-state option with three values t/nil/not-yanks
is a good customization UI then perhaps `isearch-lax-whitespace'
should provide the option `not-yanks' too, specifying whether
newlines should be translated to a single space in text yanked
into the search string.

If the value `not-yanks' is a bad UI then search functions should
take care about translating the characters in the search string internally
depending on a new set of options specifying how search functions should
use the right case and whitespace.