libpq: Handle NegotiateProtocolVersion message

libpq: Handle NegotiateProtocolVersion message

Before, receiving a NegotiateProtocolVersion message would result in a
confusing error message like

    expected authentication request from server, but received v

This adds proper handling of this protocol message and produces an
on-topic error message from it.

Reviewed-by: Jacob Champion <jchampion@timescale.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/f9c7862f-b864-8ef7-a861-c4638c83e209%40enterprisedb.com