Rollup merge of #41011 - CleanCut:bootstrap-help, r=alexcrichton

Overhaul Bootstrap (x.py) Command-Line-Parsing & Help Output

While working on #40417, I got frustrated with the behavior of x.py and the bootstrap binary it wraps, so I decided to do something about it.  This PR should improve documentation, make the command-line-parsing more flexible, and clean up some of the internals.  No command that worked before should stop working.  At least that's the theory. :-)

This should resolve at least #40920 and #38373.

Changes:

- No more manual args manipulation -- getopts used everywhere except the one place it's not possible.  As a result, options can be in any position, now, even before the subcommand.
- The additional options for test, bench, and dist now appear in the help output.
- No more single-letter variable bindings used internally for large scopes.
- Don't output the time measurement when just invoking `x.py` or explicitly passing `-h` or `--help`
- Logic is now much more linear.  We build strings up, and then print them.
- Refer to subcommands as subcommands everywhere (some places we were saying "command")
- Other minor stuff.

@alexcrichton This is my first PR. Do I need to do something specific to request reviewers or anything?

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