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.

src/shared/meson.build

@@ -94,6 +94,7 @@ shared_sources = files(
         'gnutls-util.c',
         'gpt.c',
         'group-record.c',
+        'help-util.c',
         'hibernate-util.c',
         'hostname-setup.c',
         'hwdb-util.c',