pgsql: Clean up ts_locale.h/.c.

Log Message:
-----------
Clean up ts_locale.h/.c.  Fix broken and not-consistent-across-platforms
behavior of wchar2char/char2wchar; this should resolve bug #3730.  Avoid
excess computations of pg_mblen in t_isalpha and friends.  Const-ify
APIs where possible.

Modified Files:
--------------
    pgsql/src/backend/tsearch:
        ts_locale.c (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_locale.c?r1=1.2&r2=1.3)
        ts_utils.c (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_utils.c?r1=1.4&r2=1.5)
        wparser_def.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/wparser_def.c?r1=1.7&r2=1.8)
    pgsql/src/include/tsearch:
        ts_locale.h (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tsearch/ts_locale.h?r1=1.2&r2=1.3)
        ts_public.h (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/tsearch/ts_public.h?r1=1.4&r2=1.5)