CMD: Limit the command name size to avoid big recursive calls

We do that because an ill-intentioned client might send a 2000 character
long command name to try to SEGV the lektord process because of a stack
to small to handle all the recursive calls.

The recursive calls are done in the trie find function, where the stack
consumed by each calls increases each time a recursive call is done
(indexes and other local variables...).