Provide a cleanup function for the term_tab module, freeing memory

Provide a cleanup function for the term_tab module, freeing memory
and resetting the internal state to the initial state.
Call this function from the proper place in term_free().

With the way the module is currently used, this does not imply any
functional change, but doing proper cleanup is more robust, makes
it easier during code review to understand what is going on, and
makes it explicit that there is no memory leak.