Adds documentation for various functions, removes unused functions (a
couple of simple, very lightly used functions were unDRYed; these could
go back the other way across all shell scripts if necessary). TODO items
noted for follow-up refactoring.
Fixes: #14275
Relates-to: #14269
Build-bot: build all
Test-bot: skip
Fixes up scripts (except under /linux) to use `#!/usr/bin/env bash`
instead of `#!/bin/bash` or `#!/bin/sh` so that we don't end up with
the ancient version of bash supplied with macOS.
This became urgent with this PR, because of bash-4.xisms in
build-utils.sh, for example on line 572:
```
if [[ -v _builder_params[$e] ]]; then
```
