dbi-link - dbi-link: - Added new function

Log Message:
-----------
- Added new function dbi_link.prepend_to_search_path(TEXT) which does
what it sounds like, only it has the nice feature of being
idempotent, which is to say that subsequent calls don't do more than
the first one did.

- Modified the rest of dbi_link to use it along with the examples.

Modified Files:
--------------
dbi-link:
dbi_link.sql (r1.17 -> r1.18)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/dbi_link.sql.diff?r1=1.17&r2=1.18)
dbi-link/examples/cache:
cache.sql (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/cache/cache.sql.diff?r1=1.1&r2=1.2)
dbi-link/examples/excel:
excel.sql (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/excel/excel.sql.diff?r1=1.1&r2=1.2)
dbi-link/examples/mssql:
mssql.sql (r1.2 -> r1.3)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/mssql/mssql.sql.diff?r1=1.2&r2=1.3)
dbi-link/examples/mysql:
mysql.sql (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/mysql/mysql.sql.diff?r1=1.1&r2=1.2)
dbi-link/examples/oracle:
oracle.sql (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/oracle/oracle.sql.diff?r1=1.1&r2=1.2)
dbi-link/examples/postgresql:
postgresql.sql (r1.1 -> r1.2)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/dbi-link/dbi-link/examples/postgresql/postgresql.sql.diff?r1=1.1&r2=1.2)