net: add addrList

This CL adds a new type addrList that will carry a short list of IP
addresses to dial helper functions in the upcoming CLs.

This is in preparation for TCP connection setup with fast failover on
dual IP stack node as described in RFC 6555.

Update #3610
Update #5267

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13241046

src/pkg/net/ipsock.go

@@ -40,6 +40,24 @@ type netaddr interface {
 	toAddr() Addr
 }
 
+// An addrList represents a list of network endpoint addresses.
+type addrList []netaddr
+
+func (al addrList) toAddr() Addr {
+	switch len(al) {
+	case 0:
+		return nil
+	case 1:
+		return al[0].toAddr()
+	default:
+		// For now, we'll roughly pick first one without
+		// considering dealing with any preferences such as
+		// DNS TTL, transport path quality, network routing
+		// information.
+		return al[0].toAddr()
+	}
+}
+
 var errNoSuitableAddress = errors.New("no suitable address found")
 
 // firstFavoriteAddr returns an address that implemets netaddr