net: allow "" as a hint wildcard to Resolve{IP,UDP,TCP}Addr

Also fixes comments on ResolveIPAddr.

Fixes #4476.

R=bradfitz, golang-dev
CC=golang-dev
https://golang.org/cl/6854129
diff --git a/src/pkg/net/tcp_test.go b/src/pkg/net/tcp_test.go
index e8588c6..bca7488 100644
--- a/src/pkg/net/tcp_test.go
+++ b/src/pkg/net/tcp_test.go
@@ -130,6 +130,9 @@
 	{"tcp", "[::1]:1", &TCPAddr{IP: ParseIP("::1"), Port: 1}, nil},
 	{"tcp6", "[::1]:65534", &TCPAddr{IP: ParseIP("::1"), Port: 65534}, nil},
 
+	{"", "127.0.0.1:0", &TCPAddr{IP: IPv4(127, 0, 0, 1), Port: 0}, nil}, // Go 1.0 behavior
+	{"", "[::1]:0", &TCPAddr{IP: ParseIP("::1"), Port: 0}, nil},         // Go 1.0 behavior
+
 	{"http", "127.0.0.1:0", nil, UnknownNetworkError("http")},
 }