pgsql: Have text search thesaurus files use "?" for stop words.

Log Message:
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Have text search thesaurus files use "?" for stop words.

Throw an error for actual stop words, rather than a warning.  This fixes
problems with cache reloading causing warning messages.

Re-enable stop words in regression tests;  was disabled by Tom.

Document "?" as API change.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        textsearch.sgml (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/textsearch.sgml?r1=1.30&r2=1.31)
    pgsql/src/backend/tsearch:
        dict_thesaurus.c (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/dict_thesaurus.c?r1=1.5&r2=1.6)
        thesaurus_sample.ths (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/thesaurus_sample.ths?r1=1.2&r2=1.3)
    pgsql/src/test/regress/expected:
        tsdicts.out (r1.3 -> r1.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/tsdicts.out?r1=1.3&r2=1.4)