FreeBSD/Dragonfly and Solaris have identical implementations of
sendFile. Keep one and adjust the comments accordingly.

Change-Id: I77b0f88a4816dd6e40f5cb33919c44606401ac6b
Reviewed-on: https://go-review.googlesource.com/104915
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>