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
This requires a cache-invalidation hack borrowed from cloudlands mod.
Also, null reference fixes in ensure_remote_portal_then_teleport and locate_or_build_portal, and minor improvements
Fix bug where ensure_remote_portal_then_teleport() converted its destination_wormholePos to an anchorPos to pass to locate_or_build_portal() which converted it back to an wormholePos. Bug occurred when local_orientation was assumed for the target orientation, which could lead to locate_or_build_portal() ending up with a reconstituted wormholePos rotated outside the portal frame, not find a portal frame, so build a new portal there, griefing the old one.
locate_or_build_portal() has been refactored to take a wormholePos instead of an anchorPos, removing the need to know the orientation of the target portal (which can't be obtained from the param2 when the target portal is unlit)
This is effectively a missing part to the change that made locate_or_build_portal() enforce that portals only link together in mutual pairs. As target portals that didn't need to be located or built we not being updated to link back to where the player travelled from.
Also
* implements events: on_run_wormhole, on_extinguish
* adds a lot of debug info
