systemctl: link to all non-man-page files in help

systemctl: link to all non-man-page files in help

For file:// links, we urlify the link so that the user can click and either
open the file in a editor or some viewer. The detection is chosen via some
mechanism implemented by the terminal emulator. This seems too DTRT for text
files and PDFs, which should cover the majority of realistic cases. If the file
is not viable, the terminal emulator will say
  "Could not open file://…. No application is registered to view this file type."
or similar.

For all other links, which are primarily http:// and https://, we just show the
link, letting the terminal handle the hyperlinking. The user can then ctrl-click
and open the file it their browser. If we tried to open the files automatically,
we'd would need to open many pages, and we'd need to figure out what browser to
use, etc. When the user picks whether to open the file, this leads to a nicer
user experience.

Man pages are separated by an empty line from preceding in and following output.
In my testing, this makes the output easier to read. A bit of explicit flushing
is needed to make sure that various outputs are not interleaved.

Fixes https://github.com/systemd/systemd/issues/29061.