[ticket/11288] Search fooled by hyphens.

When searching posts with keywords joined with '-' no results were
found even though there are matching posts. This is because trying
to search hyphen joined keywords as a single keyword. But 'search
_wordlist' table does not contain them as a single string joined with
hyphen. Extracted words separated from '-' as two keywords and display
search results.

PHPBB3-11288