pgsql: Add prefix support for synonym dictionary

Log Message:
-----------
Add prefix support for synonym dictionary

Modified Files:
--------------
    pgsql/doc/src/sgml:
        textsearch.sgml (r1.52 -> r1.53)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/textsearch.sgml?r1=1.52&r2=1.53)
    pgsql/src/backend/tsearch:
        dict_synonym.c (r1.10 -> r1.11)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/dict_synonym.c?r1=1.10&r2=1.11)
        synonym_sample.syn (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/synonym_sample.syn?r1=1.1&r2=1.2)
    pgsql/src/test/regress/expected:
        tsdicts.out (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/tsdicts.out?r1=1.5&r2=1.6)
    pgsql/src/test/regress/sql:
        tsdicts.sql (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/tsdicts.sql?r1=1.2&r2=1.3)