net: disable sendfile on Solaris for now

There are reports of corruption. Let's disable it for now (for Go 1.6,
especially) until we can investigate and fix properly.

Update #13892

Change-Id: I557275e5142fe616e8a4f89c00ffafb830eb3b78
Reviewed-on: https://go-review.googlesource.com/18540Reviewed-by: Dave Cheney <dave@cheney.net>

src/net/sendfile_solaris.go

@@ -26,6 +26,8 @@ const maxSendfileSize int = 4 << 20
 //
 // if handled == false, sendFile performed no work.
 func sendFile(c *netFD, r io.Reader) (written int64, err error, handled bool) {
+	return // Solaris sendfile is disabled until Issue 13892 is understood and fixed
+
 	// Solaris uses 0 as the "until EOF" value. If you pass in more bytes than the
 	// file contains, it will loop back to the beginning ad nauseam until it's sent
 	// exactly the number of bytes told to. As such, we need to know exactly how many