Fixed Portal and Worlds commands.

`/portal` now accepts zero parameters, returns the current world name in such a case
`/worlds` reports the current world name as the last output line.
Fixed the code for better Lua performance (avoid needless table-counting)
Nope, that's actually slower, because it has to count the number of elements already present in the table, while my code *knows* the number, thus doesn't need to calc