help-util: add helpers for generating uniform --help texts

help-util: add helpers for generating uniform --help texts

Let's introduce some helpers for generating uniform --help texts with
some minimal ANSI styling.

This shortens the help() functions generally, and allows us to change
the style at a single, central place.

This mostly just follows our current styling for --help, but it makes
two updates to it:

1. The command line summary at the very top of the --help text is now
   prefixes with a grey ">" character to indicate it's a command line.

2. The human language introductionary description/abstract right after
   that command line is set in italics, to emphasize it's not dry,
   technical, structural information, but more human friendly prose.