Disable SSL when connecting over a UNIX domain socket

Previously if sslmode was not explicitly disabled, trying to connect
over a UNIX domain socket would result in a "pq: SSL is not enabled on
the server" error.  This is both silly and incompatible with libpq.  Fix
by setting "sslmode" to "disable" automatically whenever a connection is
made over a UNIX socket.

conn.go

@@ -221,6 +221,10 @@ func DialOpen(d Dialer, name string) (_ driver.Conn, err error) {
 
 func dial(d Dialer, o values) (net.Conn, error) {
 	ntw, addr := network(o)
+	// SSL is not necessary or supported over UNIX domain sockets
+	if ntw == "unix" {
+		o["sslmode"] = "disable"
+	}
 
 	// Zero or not specified means wait indefinitely.
 	if timeout := o.Get("connect_timeout"); timeout != "" && timeout != "0" {