Bug#1121644: luanti-server: Add support for /usr/local/ for user provided content

[Chris Barry]

Package: luanti-server
Version: 5.10.0+dfsg-5
Severity: wishlist
X-Debbugs-Cc: chris at barry.im

Dear Maintainer,

In order to ensure a clean separation between debian provided games/mods and user provided games/modes, I suggest the following:

- User provided /usr/local/share/luanti/{games,mods}
- Debian provided /usr/share/luanti/{games,mods} 

I have this working locally by updating environment variables as follows in a systemd dropin file. These could be added to the provided systemd service files.

```
Environment=MINETEST_MOD_PATH=/usr/local/share/luanti/mods/:/usr/share/luanti/mods/
Environment=MINETEST_GAME_PATH=/usr/local/share/luanti/games/:/usr/share/luanti/games/
```

There should be considerations for the existing MINETEST_MOD_PATH since it currently uses the old name minetest (i.e. /usr/share/games/minetest/mods)

This will help server operators use debian provided packages when available while still being able to bring in their own custom modifications

-- System Information:
Debian Release: 13.2
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 6.12.48+deb13-amd64 (SMP w/4 CPU threads; PREEMPT)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8), LANGUAGE not set
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages luanti-server depends on:
ii  libc6                      2.41-12
ii  libcurl4t64                8.14.1-2+deb13u2
ii  libgcc-s1                  14.2.0-19
ii  libgmp10                   2:6.3.0+dfsg-3
ii  libjsoncpp26               1.9.6-3
ii  libleveldb1d               1.23-5+b2
ii  libluajit-5.1-2            2.1.0+openresty20250117-2
ii  libpq5                     17.6-0+deb13u1
ii  libprometheus-cpp-core1.0  1.0.2-2+b3
ii  libprometheus-cpp-pull1.0  1.0.2-2+b3
ii  libspatialindex8           2.1.0-1
ii  libsqlite3-0               3.46.1-7
ii  libstdc++6                 14.2.0-19
ii  libzstd1                   1.5.7+dfsg-1
ii  luanti-data                5.10.0+dfsg-5
ii  luanti-game-minetest       5.8.0.40.gc7be7c1-3
ii  zlib1g                     1:1.3.dfsg+really1.3.1-1+b1

luanti-server recommends no packages.

luanti-server suggests no packages.

-- Configuration Files:
/etc/luanti/default.conf changed [not included]

-- no debconf information