Add a Way of checking for specific Feature with Lua Adds minetest.get_feature() and minetest.has_feature()

When accidentally misspelling a name, privileges are sometimes granted to non-existent players, leaving them with the extra privileges, but without the default privileges, if they ever join the server. This corrects that by disallowing /grant from working with invalid players. For completeness, it does the same for /revoke.

When first entering an area, sometimes placing nodes replaces other nodes that are not buildable_to. This seems to be caused by the fact that nodes in unloaded map blocks are treated as ignore, a node that is buildable_to. This fixes that, by using get_node_or_nil() instead of the previously-used get_node(), then checking to see if the nodes were actually loaded before replacing.

Drop nodes as items when dugged and no room in inventory and dont remove dropped items when no room in inventory

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.

Scripts can call minetest.register_on_shutdown() to register a
shutdown hook.

Document that minetest.register_on_shutdown() callbacks may not be run

If the server crashes, it is unlikely that callbacks registered using
minetest.register_on_shutdown() will be called.

This reverts commit ffad18e4.

According to #253, using `/clearpassword` without an argument causes the server to crash from an assertion failure. I've resubmitted matttpt's patch as a pull request to aid in merging.