pgsql: When a cursor is opened using dblink_open, only start a

Log Message:
-----------
When a cursor is opened using dblink_open, only start a transaction
if there isn't one already open. Upon dblink_close, only commit
the open transaction if it was started by dblink_open, and only
then when all cursors opened by dblink_open are closed. The transaction
accounting is done individually for all named connections, plus
the persistent unnamed connection.

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c.diff?r1=1.47&r2=1.48)
    pgsql/contrib/dblink/expected:
        dblink.out (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/expected/dblink.out.diff?r1=1.15&r2=1.16)
    pgsql/contrib/dblink/sql:
        dblink.sql (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/sql/dblink.sql.diff?r1=1.14&r2=1.15)