os, net, internal/poll: combine unix sendfile implementations

The internal/poll/sendfile_{bsd,linux,solaris}.go implementations
have more in common than not. Combine into a single sendfile_unix.go.

The net and os packages have redundant code dealing with sendfile
quirks on non-Linux Unix systems, such as the need to determine the
size of the source file before sending. Move the common code into
internal/poll.

Remove some obsolete or incorrect behaviors:

Drop the maximum sendfile chunk size. If we ask the kernel
to copy more data than it is willing to send, it'll copy up to
its limit.

There was a comment in net/sendfile_unix_alt.go indicating that
copying more bytes than a file contains results in the kernel
looping back to the start of the file. I am unable to replicate
this behavior anywhere. Dropped the comment, the workarounds,
and added a test covering this case.

Darwin, Dragonfly, and FreeBSD all support copying the entire
contents of a file by passing 0 for the copy limit.
Take advantage of this.

Change-Id: I9f707ac7a27c165020ae02a6b5bb8f6f16f3c530
Reviewed-on: https://go-review.googlesource.com/c/go/+/621416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

0 files changed, 0 insertions, 0 deletions