Fix #11644: Off by one error/buffer over-read in StrMakeValid (#11645)

* Fix #11644: Off by one error in StrMakeValid UTF-8 decode overrun detection

* Fix #11644: Off by one error in StrMakeValid buffer last character

* Fix: Unnecessary string duplication at StrMakeValid call sites

Fix 233aac5: Set newline default comment for new groups. (#11411)

The newline space between ini groups is actually recorded as a comment of the group. This got inadvertantly dropped in #11364.

Codechange: Add const versions of GetItem/GetGroup, and sprinkle liberally.

Non-const version of GetItem is not needed.

Codechange: Use std::list instead of C-linked list for ini files.

This removes self-management of memory with new/delete and allows simpler iteration.

Codechange: Remove create parameter from IniLoadFile::GetGroup.

GetGroup now only returns nullptr if the group does not exist.
Use GetOrCreateGroup to create a group.

This avoids creating groups while reading ini files.

Codechange: Split GetGroup into GetGroup/GetOrCreateGroup.

This follows the pattern used for GetItem/GetOrCreateItem, and allows use
of references where we know the group must exist.

Codechange: Add CreateGroup/CreateItem methods for ini files.

This abstracts the internals a bit.

Codechange: Split GetItem with GetOrCreateItem. (#10952)

`IniGroup::GetItem()` returns nullptr if the item does not exist, but does not if the create parameter is set to true. Resolve CodeQL warnings with `GetOrCreateItem()` which returns a reference to the item instead.

Change: move sensitive information to secrets.cfg and private information to private.cfg

We often ask people for their openttd.cfg, which now includes their
passwords, usernames, etc. It is easy for people to overlook this,
unwillingly sharing information they shouldn't.

By splitting this information over either private.cfg or secrets.cfg,
we make it more obvious they shouldn't be sharing those files, and
hint to what is inside them.

Codechange: rename str_validate to StrMakeValid(InPlace) (#9304)

This to be more explicit the function changes the value, and not
returns yes/no.