pgsql: Separate prepared statement and bind parameters with comma.

Log Message:
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Separate prepared statement and bind parameters with comma.

Fix printing of NULL bind parameters, use "NULL".

Modified Files:
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pgsql/src/backend/tcop:
postgres.c (r1.501 -> r1.502)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.501&r2=1.502)