Made constant names more idiomatic,
moved some constants to function seedrand,
and found better name for _M.

Change-Id: I192172f398378bef486a5bbceb6ba86af48ebcc9
Reviewed-on: https://go-review.googlesource.com/107135Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>