Bug squashes and a new guide

Solved bugs and problems:

• Do not ignore the portal code if damage is enabled and the "nether_players" file does not exist.
  The bug was discovered thanks to the people who reported it on the Minetest forum.
• The nether guide is readable again (at least for me)
• Fix portal player y pos being off by 1 node sometimes (by Desour)
• Removal of unneeded dependencies and some deprecation messages

New features:

• The nether guide contains more information now and is written with the hypertext formspec element.
• A few settings are now configurable in Minetest's Advanced Settings, e.g. the log level and player trapping.

Compatibility breakages

• The nether traps players now when the nether.trap_players setting is enabled and not Minetest's enable_damage setting. This may require manual intervention on existing servers.