search_wordmatch unique key name is too long

The unique key for the search_wordmatch table, by default, is phpbb_search_wordmatch_unq_mtch, which is 31 characters long. This prevents it from being installed on Oracle since it has a limit of 30 characters. Oracle's key was previously named u_phpbb_unq_mtch. At the moment, automated tests will not run since they use the default prefix of phpbb_.