ssh: Support multiple source-addresses, don't require IPv4 in tests.

The ssh tests currently require 127.0.0.1 to work which isn't
necessarily available everywhere. To fix the source-address tests,
support comma-separated source-address values per the PROTOCOL.certkeys
file:

  Comma-separated list of source addresses
  from which this certificate is accepted
  for authentication. Addresses are
  specified in CIDR format (nn.nn.nn.nn/nn
  or hhhh::hhhh/nn).
  If this option is not present then
  certificates may be presented from any
  source address.

Change-Id: I87536ff81ffa005c073da103021ebc0dfb12b620
Reviewed-on: https://go-review.googlesource.com/36110
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>

ssh/handshake_test.go

@@ -40,7 +40,7 @@ func (t *testChecker) Check(dialAddr string, addr net.Addr, key PublicKey) error
 // therefore is buffered (net.Pipe deadlocks if both sides start with
 // a write.)
 func netPipe() (net.Conn, net.Conn, error) {
-	listener, err := net.Listen("tcp", "127.0.0.1:0")
+	listener, err := net.Listen("tcp", ":0")
 	if err != nil {
 		return nil, nil, err
 	}