Ensure that all supported options are listed by --help

[tautschnig]

Even disregarding deprecated ones, there are numerous options that are parsed by various *_parse_options.cpp files, but aren't listed in their respective help() functions.

Ideally there were an automated check to avoid such problems to re-enter the code base.

[martin-cs]

At one point @DanielNeville had a list of all of them and what they do...

[peterschrammel]

To reduce noise I'm in favour of a basic --help option and a complete --help-full (or any other suitable name).

[TGWDB]

This is a goal to achieve indefinitely. I'm closing this as it's extremely old, but if there are specific missing options now/in future please create an appropriate issue/PR.