together with explanation why we need it.

Check the theory that optimize_wo_join_buffering() doesn't need to save/restore join->cur_sj_inner_tables.

- Address review feedback in select_describe().

Make first_linear_tab()/next_linear_tab() use named constants instead of TRUE/FALSE flags, for better readability.

MWL#90: Address review feedback: fix a problem introduced a few csets earlier: Do perform tab->cleanup()
for const tables in JOIN::cleanup().

- among everything else, switch to using first/next_linear_tab() in make_outerjoin_info().

- In join buffering code, call join_tab_execution_startup() (#1) before we call join_tab_scan->open() (#2).
  This is important with SJ-Materialization because #1 fills the materialized table, while
  #2 will actually try to read the first row. Attempt to read the first row before we have
  populated the materialized table would cause zero rows to be returned when actually there were matches.

- Make equality-substitution-for-ref-access code in JOIN::optimize() treat join_tab->ref.key_copy correctly
  (in the way create_ref_for_key() has filled it).

The bug in the function print_keyuse() caused crashes if
hash join could be used. It happened because the function
ignored the fact that KEYUSE structures could be created
for hash joins as well. 

sql/mysqld.cc:
  Fixed: optimize_join_buffer_size was missing in the description of possible
  options for the optimizer switch.
sql/sql_select.cc:
  Fixed: initialization for the field ref_table_rows of the KEYUSE structure was 
  missing (as a result of a lame merge).

- Make get_constant_key_infix() take into account that there may be SEL_TREEs with
  type=SEL_ARG::MAYBE_KEY, which it cannot process, because they are not real ranges
  but rather indications that we might have been able to construct a range if we had
  values for some other tables' fields. 
  (check_quick_select() already has such check)