postgresql plugin: Added support for protocol versions less than 3.

postgresql plugin: Added support for protocol versions less than 3.

Support for passing parameters to a query was added in protocol version 3
which was introduced in version 7.4 of PostgreSQL (later version still support
earlier protocol versions though). If that is not available, we now fall back
to PQexec() if no parameters have been specified. Else, we skip the query and
report an error.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>