Switch mapgen to using biomes (still with backwards compatibility)

[Treer]

This moves the original mapgen code to mapgen_nobiomes.lua, and provides an MT5 biomes-based equivalent mapgen in mapgen.lua. Both mapgens use paramat's cave noise. The old mapgen is loaded in v6 worlds and versions of MT prior to 5.1

My own reason for the change is I want to add a lot to the Nether, and getting it working with biomes API, ores API etc provides a better platform to build on.

However, this PR provides many other advantages:

• mapgen is ~20% faster ...(at least until more features get added).
• Dungeons are preserved. It's not perfect, but the original code was converting them to netherbrick, followed shortly by overwriting entirely with netherrack and losing them.
• Nether has both a min height and max height, to play nicely with other mods that create realms in underground y ranges. These heights are configurable in the settings.
• Glowstone/lava scarcities are adjustable without affecting other biomes/mapgen.
• Glowstone and netherrack stalactites
• Other mods can add ores etc to the Nether. This is something I'd like for allowing game progression, e.g. to obtain the material for <goal> (or to build a portal to <other goal>) you must first reach and journey through the Nether. As though to underscore this, I hear that for the first time Mojang will add a substance harder than diamond, and they're placing it as an ore in the Nether. A biomes mapgen means Minetest mods have that option.
• Unique decoration and slightly incomplete excavation of dungeons / occasional happy accidents of the excavator, gives Nether dungeons a distinct feel, different from traditional Minetest dungeons.

Things to note:

• override_underground_biomes() will shift biomes that overlap the Nether, this includes many of the MTG biomes such as "savanna_under" which extend from -256 to -31000
• By default, the Nether now extends from -5000 to -11000 instead of all the way to -32000. This might have been what @FaceDeer was concerned about (?) However PilzAdam's portal started searching from the top of the Nether (starting randomly between 5500 - 6500) and normally gives up after 4096, so it seems unlikely that people ever had ways of reaching very deep nether.

Tips to make evaluating the changes easier:

• diff mapgen_nobiomes.lua against the original mapgen.lua, rather than trying to vet it as if it were new code.
• diff mapgen.lua against mapgen_nobiomes.lua, and note that find_nether_ground_y and on_gerated are mostly the same, with on_gerated no longer needing to iterate through the overdraw regions to remove lava and water (due to node_cave_liquid = "air" in the nether_caverns biome definition). Vet normally the new code in mapgen.lua for setting up biomes and excavating dungeons.
• mapgen_decorations and the init can be checked as you usually would.
• Optionally confirm that Nether generated with old map is seamless with the biomes-based changes.
• Consider if the "things to note" section above raise any concerns

Git flow?

Rather than end up with a massive PR like the Portals API, would the maintainers of this mod prefer a dev branch, and approve/merge in smaller PRs like this one into it. Then when I'm ready for the changes to go into master there's no review/testing needed?

(though I'm also quite happy to just keep adding to a branch and have a massive PR at the end)

[SmallJoker]

@Treer nether is now under your control. Please feel free to rebase and merge this PR if this wasn't already implemented by one of the previous massive PRs.