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/fd_unix.go b/src/pkg/net/fd_unix.go
index 3b67b79..7a8d7d3 100644
--- a/src/pkg/net/fd_unix.go
+++ b/src/pkg/net/fd_unix.go
@@ -243,7 +243,7 @@
 	return
 }
 
-func (fd *netFD) ReadFrom(p []byte) (n int, sa syscall.Sockaddr, err error) {
+func (fd *netFD) readFrom(p []byte) (n int, sa syscall.Sockaddr, err error) {
 	if err := fd.readLock(); err != nil {
 		return 0, nil, err
 	}
@@ -270,7 +270,7 @@
 	return
 }
 
-func (fd *netFD) ReadMsg(p []byte, oob []byte) (n, oobn, flags int, sa syscall.Sockaddr, err error) {
+func (fd *netFD) readMsg(p []byte, oob []byte) (n, oobn, flags int, sa syscall.Sockaddr, err error) {
 	if err := fd.readLock(); err != nil {
 		return 0, 0, 0, nil, err
 	}
@@ -341,7 +341,7 @@
 	return nn, err
 }
 
-func (fd *netFD) WriteTo(p []byte, sa syscall.Sockaddr) (n int, err error) {
+func (fd *netFD) writeTo(p []byte, sa syscall.Sockaddr) (n int, err error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, err
 	}
@@ -366,7 +366,7 @@
 	return
 }
 
-func (fd *netFD) WriteMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int, oobn int, err error) {
+func (fd *netFD) writeMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int, oobn int, err error) {
 	if err := fd.writeLock(); err != nil {
 		return 0, 0, err
 	}