Allow to change (or disable) the default driver name for registration

[demoManito]

Description

A link variable now allows to change or disable the name of the driver that is automatically registered with database/sql.
This allows users to give the postgres name to another database/sql driver.

The implementation is the same as in: https://github.com/go-sql-driver/mysql driver. See https://github.com/go-sql-driver/mysql/blob/af8d7931954ec21a96df9610a99c09c2887f2ee7/driver.go#L92

Usage

Change the driver name to custom:

go build "-ldflags=-X github.com/lib/pq.driverName=custom"

Disable the automatic driver registration (set driverName to an empty string):

go build "-ldflags=-X github.com/lib/pq.driverName="

In the same way, a variable overridable at link time is also provided to override the driver name used in the test suite. This allows to run our test suite on another driver.

go build "-ldflags=-X github.com/lib/pq.driverNameTest=custom"

driverName is propagated to driverNameTest unless driverNameTest is explicitely defined.

[arp242]

Seems okay at a glance, but what is the use case for this? It's not mentioned in the PR, or in the PR for the MySQL driver (go-sql-driver/mysql#1499).

[arp242]

I'm closing this for now as it's unclear what problem this addresses or why we want to do this, so it's also not clear if this is the best way to address it.

Can reopen if more details are added.