On dragonfly, freebsd and solaris the sendfile syscall does not update
the read position of the source fd. Update it after sendfile so
successive calls start at the correct position.

Fixes #25809

Change-Id: Iaac79f89704b75b8038d4bb60eaf793a262cdd8f
Reviewed-on: https://go-review.googlesource.com/117895
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>