Use SyscallConn to avoid calling the Fd method in sendFile on Unix
systems, since Fd has the side effect of putting the descriptor into
blocking mode.

Fixes #28330

Change-Id: If093417a225fe44092bd2c0dbbc3937422e98c0b
Reviewed-on: https://go-review.googlesource.com/c/155137
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>