Make sure that SELECT_LEX_UNIT::derived, behaves as documented
(points to the "TABLE_LIST representing this union in the
embedding select"). For recursive CTE this was not necessarily
the case, it could've pointed to the TABLE_LIST inside the CTE,
not in the embedding select.

To fix:
* don't update unit->derived in mysql_derived_prepare(), pass derived
  as an argument to st_select_lex_unit::prepare()
* prefer to set unit->derived in TABLE_LIST::init_derived()
  to the TABLE_LIST in the embedding select, not to the recursive
  reference. Fail if there are many TABLE_LISTs in the embedding
  select with conflicting FOR SYSTEM_TIME clauses.

cleanup:
* remove redundant THD* argument from st_select_lex_unit::prepare()