pgsql: Remove use of '<' and '>' in SGML, use '&' escapes.

Log Message:
-----------
Remove use of '<' and '>' in SGML, use '&' escapes.

Update find_gt_lt to allow grep parameters to be passed into it.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        ddl.sgml (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml.diff?r1=1.64&r2=1.65)
        plpython.sgml (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpython.sgml.diff?r1=1.33&r2=1.34)
    pgsql/doc/src/sgml/ref:
        create_opclass.sgml (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_opclass.sgml.diff?r1=1.17&r2=1.18)
    pgsql/src/tools:
        find_gt_lt (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tools/find_gt_lt.diff?r1=1.2&r2=1.3)