cmd/dist: rename variables + functions

This belongs to a series of clean-up changes (see below) for cmd/dist.
This is change (9).

These changes include:
(1)  apply minor fixes
(2)  restore behavior of branchtag
(3)  unleash bootstrap optimization for windows
(4)  use standard generated code header
(5)  remove trivial variables + functions
(6)  move functions for the better
(7)  simplify code segments
(8)  use bytes.Buffer for code generation
(9)  rename variables + functions

Change-Id: I9247433d7d07a2c99d15b0a4d23164bcbc042768
Reviewed-on: https://go-review.googlesource.com/61015


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>