Also move it to static, as nobody else is using it.

…e generic

Future additions will start using it for std::list too.

…part

You can easily mistake SlList / SL_LST to be a list of SL_VAR, but
it is a list of SL_REF. With this rename, it hopefully saves a few
people from "wtf?" moments.

…D#9343)

This is especially useful for automated-testing, to make a save
when the game quits while using "-vnull:ticks=N".

…e to use vector/string

…llocations

…stom allocation

…nTTD#9342)

…and terminated when being read from the savegame

…variable (OpenTTD#9330)

…#9341)

This is extreme useful for automated testing. Without this, OpenTTD
will always look in your personal-dir (like ~/.local/share/openttd
or %USER%\Documents\OpenTTD). For most users this is exactly what
we want, that there is a shared place for all their files.

However, for automated testing this is rather annoying, as your
local development files influence the automated test. As such,
'-X' counters this, and only gives the local folders. This is
especially useful in combination with '-x' and '-c'.

…imum version