Commands that need to modify the running user structure defined in scli.h
should reside here. They (will) have a slightly different prototype that
allows passing the user structure to them for ease of modifications.

Examples of what should be a built-in and not a module would be:

cd     (cliuser_t->cwd needs to be updated)

In the future, more user preferences will be set via built-in commands,
such as the formatting of the prompt string (HelenOS doesn't yet have
an environment, much less PS*, even if it did we'd likely do it a little
differently).

.... etc.

Anything that does _not_ need to use this structure should be included
as a module, not a built in. If you want to include a new command, there
is a 99% chance that you want it to be a module.