net: move InvalidAddrError type into net.go

Probably we should remove this type before Go 1 contract has settled,
but too late. Instead, keep InvalidAddrError close to package generic
error types.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12670044
diff --git a/src/pkg/net/net.go b/src/pkg/net/net.go
index c077b4c..fbf207c 100644
--- a/src/pkg/net/net.go
+++ b/src/pkg/net/net.go
@@ -372,6 +372,12 @@
 func (e UnknownNetworkError) Temporary() bool { return false }
 func (e UnknownNetworkError) Timeout() bool   { return false }
 
+type InvalidAddrError string
+
+func (e InvalidAddrError) Error() string   { return string(e) }
+func (e InvalidAddrError) Timeout() bool   { return false }
+func (e InvalidAddrError) Temporary() bool { return false }
+
 // DNSConfigError represents an error reading the machine's DNS configuration.
 type DNSConfigError struct {
 	Err error