libpq support for NegotiateProtocolVersion

We have the NegotiateProtocolVersion protocol message [0], but libpq 
doesn't actually handle it.

Say I increase the protocol number in libpq:

-       conn->pversion = PG_PROTOCOL(3, 0);
+       conn->pversion = PG_PROTOCOL(3, 1);

Then I get

psql: error: connection to server on socket "/tmp/.s.PGSQL.65432" 
failed: expected authentication request from server, but received v

And the same for a protocol option (_pq_.something).

Over in the column encryption patch, I'm proposing to add such a 
protocol option, and the above is currently the behavior when the server 
doesn't support it.

The attached patch adds explicit handling of this protocol message to 
libpq.  So the output in the above case would then be:

psql: error: connection to server on socket "/tmp/.s.PGSQL.65432" 
failed: protocol version not supported by server: client uses 3.1, 
server supports 3.0

Or to test a protocol option:

@@ -2250,6 +2291,8 @@ build_startup_packet(const PGconn *conn, char *packet,
     if (conn->client_encoding_initial && conn->client_encoding_initial[0])
         ADD_STARTUP_OPTION("client_encoding", 
conn->client_encoding_initial);

+   ADD_STARTUP_OPTION("_pq_.foobar", "1");
+
     /* Add any environment-driven GUC settings needed */
     for (next_eo = options; next_eo->envName; next_eo++)
     {

Result:

psql: error: connection to server on socket "/tmp/.s.PGSQL.65432" 
failed: protocol extension not supported by server: _pq_.foobar


[0]: 
https://www.postgresql.org/docs/devel/protocol-message-formats.html#PROTOCOL-MESSAGE-FORMATS-NEGOTIATEPROTOCOLVERSION