Codechange: wire "load" support into all OMANY and MMANY for settings

[TrueBrain]

Motivation / Problem

While working on #11143 I had some arguments with the code in respect to load and OMANY.

Description

This PR fixes so that load works for all OMANY (and MMANY). Nobody actually uses it, so you can argue if this PR should be merged, but the current code was silly.

While at it, also reworked the logic slightly, that it first tries to use the lookup, next the conversion, and only after that check if it is a digit. The original code did the digit first, which meant you can't do a lookup starting with a number.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[rubidium42]

Suggested change

	idx = many_cnvt(str);
	if (idx != (size_t)-1) return idx;
	if (many_cnvt != nullptr) {
	idx = many_cnvt(str);
	if (idx != (size_t)-1) return idx;
	}

As far as I can see many_cnvt can be nullptr.

[TrueBrain]

The more I look at this PR, the less I think we should do it. The new stuff isn't used, and one could argue we shouldn't be using this anymore, in favour of renaming settings to remain backwards (and forwards) compatible between versions. So I am going to close this up for now!