pgsql: Replace PLpgSQL_dstring by StringInfo.

Log Message:
-----------
Replace PLpgSQL_dstring by StringInfo.

Replace redundant PLpgSQL_dstring functionality with StringInfo.
Patch by Pavel Stehule. Review by Joe Conway.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.126 -> r1.127)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.126&r2=1.127)
        pl_exec.c (r1.245 -> r1.246)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_exec.c?r1=1.245&r2=1.246)
        pl_funcs.c (r1.79 -> r1.80)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_funcs.c?r1=1.79&r2=1.80)
        plpgsql.h (r1.113 -> r1.114)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.113&r2=1.114)