Currently selectgo is just a wrapper around selectgoImpl. This keeps
the hard-coded frame skip counts for tracing the same between the
channel implementation and the select implementation.

However, this is fragile and confusing, so pass a skip parameter to
send and recv, join selectgo and selectgoImpl into one function, and
use decrease all of the skips in selectgo by one.

Change-Id: I11b8cbb7d805b55f5dc6ab4875ac7dde79412ff2
Reviewed-on: https://go-review.googlesource.com/37860
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>