net: fix sendfile regression with io.Copy on macOS
Since CL 472475, io.Copy can no longer use sendfile on macOS for copying
files to a socket due to a too strict type assertion. This CL fixes the
issue by checking for the necessary interfaces instead of the concrete
os.File type in sendfile_unix_alt.go.
Fixes #66988
Change-Id: Ia0dd190f6575016a191c34a935132907147c8e10
Reviewed-on: https://go-review.googlesource.com/c/go/+/581035
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com>