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).