t_tune_tags() now return a deep copy instead of internal data.

  Previous behaviour could lead to nasty bugs, since one could get the
  reference of the internal tags from a t_tune with t_tune_tags() and
  then call t_tune_set_tags() which would delete the internal reference,
  but the caller would still have it. Returning a deep copy (using
  t_taglist_clone()) avoid this issue.