LEX, st_select_lex, st_select_unit optimized for space:
- Use bit fields for bool variables
- Ensure that all bit fields are initialized (improves
  performance for init functions as all bit fields can be
  initalized with one memory access)
- Move members around in above structures to remove alignment
  gaps

Some savings:
LEX: 7032 -> 6880
THD: 25608 -> 25456
st_select_lex_unit: 2048 -> 2008

LEX::start():                    1321 -> 1245 instructions
st_select_lex_unit::init_query()  284 ->  214 instructions
st_select_lex::init_query():      766 ->  692 instructions
st_select_lex::init_select():     563 ->  540 instructions

Other things:
- Removed not used LEX::select_allow_into
- Fixed MDEV-25510 Assertion `sel->select_lock ==
   st_select_lex::select_lock_type::NONE' which was caused by this commit.