Codechange: automatic adding of _t to (u)int types, and WChar to char32_t

for i in `find src -type f|grep -v 3rdparty/fmt|grep -v 3rdparty/catch2|grep -v 3rdparty/opengl|grep -v stdafx.h`; do sed 's/uint16& /uint16 \&/g;s/int8\([ >*),;[]\)/int8_t\1/g;s/int16\([ >*),;[]\)/int16_t\1/g;s/int32\([ >*),;[]\)/int32_t\1/g;s/int64\([ >*),;[]\)/int64_t\1/g;s/ uint32(/ uint32_t(/g;s/_uint8_t/_uint8/;s/Uint8_t/Uint8/;s/ft_int64_t/ft_int64/g;s/uint64$/uint64_t/;s/WChar/char32_t/g;s/char32_t char32_t/char32_t WChar/' -i $i; done

Change #10077: make maximum loan a positive multiple of the loan interval

And set the minimum maximum loan to the value of loan interval, so there is
always an amount of money to lend. Compared to being allowed to set max loan
to 0 and never be allowed to lend any money.

Fix #9386: compilers failing to compile with LTO by using variants instead of new + unique_ptr

With std::variant all memory can be figured out at compile time, so the compiler needs to keep track of fewer elements. It also saves out a unique_ptr and its memory management, over a slight impact for resolving a setting.

Codechange: remove overloading on UpdateServiceInterval

One UpdateServiceInterval has two parameters to update the service interval for a vehicle type, the other for all vehicle types at once. Rename the latter to help with function resolution for the introduction of variants.

Codechange: merge guiflags and flags in settings .ini files

It was rather confusing which one was for what, especially as some
SaveLoad flags were settings-only. Clean up this mess a bit by
having only Setting flags.

Codechange: rename SettingGuiFlag to SettingFlag (#9332)

It is a lovely organicly grown enum, where it started off with
GUI-only flags, and after that a few flags got added that can be
considered GUI-only (the GUI disables/enables based on them), to
only have flags added that has nothing to do with the GUI.

So be less confusing, and rename them to what they do.

Additionally, I took this opportunity to rename 0ISDISABLED to
reflect what it really does.