games/minetest: fix rc.d script

• Introduce minetest_gameid rc.d variable (set to minetest_game by default), which passes required --gameid to minetestserver.
• Rework minetestserver arguments handling, simplify specifying extra arguments with minetest_args.
• Add check for empty list of games, suggesting to install minetest_game port or other mods.
• Fix minetest_config variable spelling

PR: 279251
Submitted by: DtxdF@disroot.org