postgresql plugin: Added support for passing parameters to a query.

postgresql plugin: Added support for passing parameters to a query.

The parameters are referred to in the query string as $1, $2, ... The value of
a parameter is specified using the new "Param <name>" config option of a
"<Query>" block. <name> may be any of "hostname", "database" or "username"
which will be replaced by the value of the appropriate connection parameters.
The hostname will not evaluate to the the UNIX domain socket path -
"localhost" will be used instead.

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