Fix Go back-compatibility in Accept error check.

Emit the message as DEBUG, so logs avoid false positive errors.

net.ErrClosed was added in Go 1.16, but this project's go.mod specifies
Go 1.13.  We alias Go's internal error type, to avoid the
worse alternative of comparing the string of the error message.

Go maintainers discuss this topic at length in
 golang/go#4373.
Occurences of "use of closed network connection"
are not an actual problem.  It's advisory only, akin to io.EOF.

net_err_closed.go

@@ -0,0 +1,11 @@
+//go:build !go1.16
+// +build !go1.16
+
+package plugin
+
+import _ "unsafe"
+
+// NetErrClosed aliases the internal error type poll.ErrNetClosing.
+// FUTURE: When Go 1.16 is the minimum supported version for go-plugin, switch to net.ErrClosed.
+//go:linkname NetErrClosed internal/poll.ErrNetClosing
+var NetErrClosed error

net_err_closed_go1.16.go

@@ -0,0 +1,8 @@
+//go:build go1.16
+// +build go1.16
+
+package plugin
+
+import "net"
+
+var NetErrClosed = net.ErrClosed

rpc_server.go

@@ -45,7 +45,9 @@ func (s *RPCServer) Serve(lis net.Listener) {
 	for {
 		conn, err := lis.Accept()
 		if err != nil {
-			if ! errors.Is(err, net.ErrClosed) {
+			if errors.Is(err, NetErrClosed) {
+				log.Printf("[DEBUG] plugin: plugin server: %s", err)
+			} else {
 				log.Printf("[ERR] plugin: plugin server: %s", err)
 			}
 			return