API improvements

Improvements found while using the portals API in another mod (cloudlands):
* remote_portal_checkup() will check the whole portal and frame for mapgen overwrites.
* Allow find_realm_anchorPos() to return nil if no realm location could be found (portal will fail to ignite).
* Allow create_book_of_portals() to be invoked [indirectly] by other mods.

Also lowers nether_book_close.png to be 4bpp

README.md

@@ -1,10 +1,9 @@
-Nether Mod for Minetest, with Portal API.
+Nether Mod for Minetest, with Portals API.
 
 See portal_api.txt for how to create custom portals to your own realms.
 
-A Book of Portals item (nether:book_of_portals) is created which
-provides players with portal recipes, by default it can be found in
-dungeon chests.
+See settingtypes.txt or go to "Settings"->"All Settings"->"Mods"->"nether"
+in the game to view the options provided by this mod.
 
 
 ## License of source code: