In the event of a partial write on Solaris and some BSDs, the offset
pointer passed to sendfile() will be updated even though the function
returns -1 if errno is set to EAGAIN/EINTR.  In that case, calculate the
bytes written based on the difference between the updated offset and the
original offset.  If no bytes were written, and errno is set to
EAGAIN/EINTR, ignore the errno.

Fixes #13892

Change-Id: I6334b5ef2edcbebdaa7db36fa4f7785967313c2d
Reviewed-on: https://go-review.googlesource.com/21769
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>