Block all dblink functions in the SQL mode

Only dblink() and dblink_connect() were blocked, so dblink_exec(),
dblink_send_query() and the other functions of the extension could
still reach other databases. Match every function whose name starts
with dblink.

tests/Unit/Service/SqlGuardTest.php

@@ -23,6 +23,9 @@ class SqlGuardTest extends TestCase
             ["SELECT pg_ls_dir('/')"],
             ["SELECT lo_import('/etc/passwd')"],
             ["SELECT dblink_connect('host=x')"],
+            ["SELECT dblink_exec('host=x', 'DROP TABLE t')"],
+            ["SELECT dblink_send_query('c', 'SELECT 1')"],
+            ["SELECT * FROM dblink('host=x', 'SELECT 1') AS t(a int)"],
             ["SELECT * FROM t INTO OUTFILE '/tmp/x'"],
             ["COPY (SELECT '/*') TO PROGRAM 'sh' --*/"],
             ["COPY (SELECT '--') TO PROGRAM 'sh'"],