(new animation by Extex101)
The original texture is now "nether_portal_alt.png", and used to provide a "nether:portal_alt" node, which is used/demoed by the Surface-portal as the original animation still has a "it's full of stars" charm coming out when used for lighter coloured portals.
Also adds nether.register_wormhole_node(), moving the wormhole node template into portal_api.lua, and allowing wormhole nodes with custom post_effect_color to be more easily created.
When a surface level was known, only the volume_is_natural check was being performed, this fixes that.
Also fixes bug where LBM didn't start the timer of example portals that had been disabled.
Allows portal ignite to repair a stopped timer.
Other misc cleanup - paramat's new Floatlands won't have lakes so I removed that from the flavortext.
give nether_portal_circular.mts a flat floor - the new nether_portal_circular.mts allows its bottom nodes to be sunk into the ground.
Example portal basic behavior and book_of_portals_pagetext implemented.
Other changes are allowing portals corrupted by ABMs to be repaired, finding better ground level of surface portals.
UTF-8 with BOM crashes some systems according to 6551f5c120 comment, and there are several cases where Minetest is buggy with files that use CRLF line endings (though none I'm aware that affect these files), so strip CRs and BOM
