Better validate MODE +k & +l parameters and return errors

Better validate MODE +k & +l parameters and return errors

Implement new numeric ERR_INVALIDMODEPARAM_MSG(696) and:

- Reject channel keys with spaces and return ERR_INVALIDMODEPARAM_MSG;
  This was possible until now and resulted in garbled IRC commands later.
- Reject empty channel keys and return ERR_INVALIDMODEPARAM_MSG;
  This was possible until now and resulted in garbled IRC commands later.
- Return ERR_INVALIDMODEPARAM_MSG when user limit is out of bounds;
  This was silently ignored until now.

Closes #290. Thanks Val Lorentz for reporting it!