scripts: improve rendering of synopsis/usage

scripts: improve rendering of synopsis/usage

Subcommands "--help" is now rendered as:

```
 tahoe [global-options] COMMAND [options] ARGS
 (use 'tahoe --help' to view global options)
 USAGE (flags/options)
 DESCRIPTION
 DESCRIPTION_UNWRAPPED
```

The new .description and .description_unwrapped fields allow
commands (subclasses of twisted.python.usage.Usage) better control over
how their explanations are rendered: the old .longdesc field was wrapped
unpleasantly.