net,os: consolidate poll.SendFile sending until EOF with 0

We've already use size==0 to indicate sending until EOF for
poll.SendFile on non-Linux platforms: Windows/*BSD/macOS/Solaris.

Let's harmonize Linux with others, making poll.SendFile on Linux
match its comment.

Change-Id: Ibfe9c9aa8f16bc37812afce9f95995c715cce0bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/623057
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/net/sendfile_linux.go

@@ -20,7 +20,7 @@ const supportsSendfile = true
 //
 // if handled == false, sendFile performed no work.
 func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
-	var remain int64 = 1<<63 - 1 // by default, copy until EOF
+	var remain int64 = 0 // 0 indicates sending until EOF
 
 	lr, ok := r.(*io.LimitedReader)
 	if ok {

src/os/zero_copy_linux.go

@@ -28,7 +28,7 @@ func (f *File) writeTo(w io.Writer) (written int64, handled bool, err error) {
 	}
 
 	rerr := sc.Read(func(fd uintptr) (done bool) {
-		written, err, handled = poll.SendFile(pfd, int(fd), 1<<63-1)
+		written, err, handled = poll.SendFile(pfd, int(fd), 0)
 		return true
 	})
 

src/os/zero_copy_solaris.go

@@ -17,7 +17,7 @@ func (f *File) writeTo(w io.Writer) (written int64, handled bool, err error) {
 
 // readFrom is basically a refactor of net.sendFile, but adapted to work for the target of *File.
 func (f *File) readFrom(r io.Reader) (written int64, handled bool, err error) {
-	var remain int64 = 0
+	var remain int64 = 0 // 0 indicates sending until EOF
 	lr, ok := r.(*io.LimitedReader)
 	if ok {
 		remain, r = lr.N, lr.R