runtime: rename _MSpan* constants to mSpan*

We already aliased mSpanInUse to _MSpanInUse. The dual constants are
getting annoying, so fix all of these to use the mSpan* naming
convention.

This was done automatically with:
  sed -i -re 's/_?MSpan(Dead|InUse|Manual|Free)/mSpan\1/g' *.go
plus deleting the existing definition of mSpanInUse.

Change-Id: I09979d9d491d06c10689cea625dc57faa9cc6767
Reviewed-on: https://go-review.googlesource.com/137875


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>