Add user alerts and check on node digging

- Bump to version 1.2.2
 - Add `metatools.alert_users` which sends the string passed as argument
   to all players currently operating on nodes
 - Register a dignode callback in which we alert users about players
   trying to dig operated nodes, or open nodes in general. Only node
   operated by a person different from the person digging are blocked
 - Add `metatools.get_context_from_pos`. It receives a table, but does
   no assertions (yet?)
 - Move `playerlocks` over to `metatools.playerlocks` for similar
   reasons to why we moved `nodelock`
 - Create field `success` for meta_exec, a function run when the command
   is successful; it receives the executed function's parameters and its
   output (status, msg) as arguments
 - Update /meta help's message
 - Fix instances of `contexts` not replaced by `metatools.contexts`
 - Assert that the context id is correct upon running `metatools.show`

assertions.lua

@@ -5,7 +5,7 @@ function assert_contextid(ctid)
 end
 
 function assert_ownership(ctid, name)
-	return playerlocks[name] == ctid
+	return metatools.playerlocks[name] == ctid
 end
 
 function assert_pos(pos)