net: don't export netFD readFrom, writeTo, readMsg, writeMsg methods

There is no way to call them from outside the net package.
They are used to implement UCPConn.ReadMsgUDP and similar.

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/83730044
diff --git a/src/pkg/net/udpsock_posix.go b/src/pkg/net/udpsock_posix.go
index d81c253..5dfba94 100644
--- a/src/pkg/net/udpsock_posix.go
+++ b/src/pkg/net/udpsock_posix.go
@@ -64,7 +64,7 @@
 	if !c.ok() {
 		return 0, nil, syscall.EINVAL
 	}
-	n, sa, err := c.fd.ReadFrom(b)
+	n, sa, err := c.fd.readFrom(b)
 	switch sa := sa.(type) {
 	case *syscall.SockaddrInet4:
 		addr = &UDPAddr{IP: sa.Addr[0:], Port: sa.Port}
@@ -93,7 +93,7 @@
 		return 0, 0, 0, nil, syscall.EINVAL
 	}
 	var sa syscall.Sockaddr
-	n, oobn, flags, sa, err = c.fd.ReadMsg(b, oob)
+	n, oobn, flags, sa, err = c.fd.readMsg(b, oob)
 	switch sa := sa.(type) {
 	case *syscall.SockaddrInet4:
 		addr = &UDPAddr{IP: sa.Addr[0:], Port: sa.Port}
@@ -124,7 +124,7 @@
 	if err != nil {
 		return 0, &OpError{"write", c.fd.net, addr, err}
 	}
-	return c.fd.WriteTo(b, sa)
+	return c.fd.writeTo(b, sa)
 }
 
 // WriteTo implements the PacketConn WriteTo method.
@@ -156,7 +156,7 @@
 	if err != nil {
 		return 0, 0, &OpError{"write", c.fd.net, addr, err}
 	}
-	return c.fd.WriteMsg(b, oob, sa)
+	return c.fd.writeMsg(b, oob, sa)
 }
 
 // DialUDP connects to the remote address raddr on the network net,