netlink: deprecate Socket interface

Signed-off-by: Matt Layher <mdlayher@gmail.com>

conn.go

@@ -44,6 +44,10 @@ type Conn struct {
 
 // A Socket is an operating-system specific implementation of netlink
 // sockets used by Conn.
+//
+// Deprecated: the intent of Socket was to provide an abstraction layer for
+// testing, but this abstraction is awkward to use properly and disables much of
+// the functionality of the Conn type. Do not use.
 type Socket interface {
 	Close() error
 	Send(m Message) error

nltest/nltest.go

@@ -126,8 +126,6 @@ func CheckRequest(types []netlink.HeaderType, flags []netlink.HeaderFlags, fn Fu
 	}
 }
 
-var _ netlink.Socket = &socket{}
-
 // A socket is a netlink.Socket used for testing.
 type socket struct {
 	fn Func