libpq: Handle NegotiateProtocolVersion message differently

Previously libpq would always error out if the server sends a
NegotiateProtocolVersion message. This was fine because libpq only
supported a single protocol version and did not support any protocol
parameters. But in the upcoming commits, we will introduce a new
protocol version and the NegotiateProtocolVersion message starts to
actually be used.

This patch modifies the client side checks to allow a range of
supported protocol versions, instead of only allowing the exact
version that was requested. Currently this "range" only contains the
3.0 version, but in a future commit we'll change this.

Also clarify the error messages, making them suitable for the world
where libpq will support multiple protocol versions and protocol
extensions.

Note that until the later commits that introduce new protocol version,
this change does not have any behavioural effect, because libpq will
only request version 3.0 and will never send protocol parameters, and
therefore will never receive a NegotiateProtocolVersion message from
the server.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (earlier versions)
Discussion: https://www.postgresql.org/message-id/CAGECzQTfc_O%2BHXqAo5_-xG4r3EFVsTefUeQzSvhEyyLDba-O9w@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAGECzQRbAGqJnnJJxTdKewTsNOovUt4bsx3NFfofz3m2j-t7tA@mail.gmail.com

src/interfaces/libpq/libpq-int.h

@@ -496,6 +496,8 @@ struct pg_conn
 	SockAddr	raddr;			/* Remote address */
 	ProtocolVersion pversion;	/* FE/BE protocol version in use */
 	int			sversion;		/* server version, e.g. 70401 for 7.4.1 */
+	bool		pversion_negotiated;	/* true if NegotiateProtocolVersion
+										 * was received */
 	bool		auth_req_received;	/* true if any type of auth req received */
 	bool		password_needed;	/* true if server demanded a password */
 	bool		gssapi_used;	/* true if authenticated via gssapi */