1. Adding const qualifiers into a few method parameters.

2. Adding methods:
- sp_label::block_label_declare()
- LEX::sp_block_init()
- LEX::sp_block_finalize()
  to share more code between the files sql_yacc.yy and sql_yacc_ora.yy,
  as well as between the rules sp_labeled_block, sp_unlabeled_block,
  sp_unlabeled_block_not_atomic.

3. sql_yacc.yy, sql_yacc_ora.yy changes:
- Removing sp_block_content
- Reorganizing the grammar so the rules sp_labeled_block,
  sp_unlabeled_block, sp_unlabeled_block_not_atomic now
  contain both BEGIN_SYM and END keywords. Previously,
  BEGIN_SYM and END resided in different rules.
  This change makes the grammar easier to read,
  as well as simplifies adding Oracle-style DECLARE section (coming soon):
    DECLARE
      ..
    BEGIN
      ..
    END;

  Good side effects:
  - SP block related grammar does not use Lex->name any more.
  - The "splabel" member was removed from %union