dblink for 8.4 should work without user-mappings

contrib/dblink in 8.4 supports a server name by CREATE SERVER for connection
string, but it always requires an user-mapping (by CREATE USER MAPPING).
However, I think it should work user-mappings because it works when
the connection string is passed directly.

=# SELECT * FROM dblink('dbname=postgres', 'SELECT current_user') AS t(i name);
(ok)

=# CREATE FOREIGN DATA WRAPPER postgresql VALIDATOR postgresql_fdw_validator;
=# CREATE SERVER server1 FOREIGN DATA WRAPPER postgresql OPTIONS (dbname 'postgres');
=# SELECT * FROM dblink('server1', 'SELECT 1') AS t(i integer);
ERROR: user mapping not found for "postgres"

The attached patch adds 'missing_ok' parameter to GetUserMapping() and
made dblink to use it. There should be no additional security issues here
because dblink's original security check works even for server name mode.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center