MDEV-16244 sql_mode=ORACLE: Some keywords do not work in variable declarations

1. Adding LEX::make_item_sysvar() and reusing it
   in sql_yacc.yy and sql_yacc_ora.yy.
   Removing the "opt_component" rule.

2. Renaming rules to better reflect their purpose:
   - keyword to keyword_ident
   - keyword_sp to keyword_label
   - keyword_sp_not_data_type to keyword_sp_var_and_label

   Also renaming:
   - sp_decl_ident_keyword to keyword_sp_decl for naming consistency
   - keyword_alias to keyword_table_alias,
     for consistency with ident_table_alias
   - keyword_sp_data_type to keyword_data_type,
     as it has nothing SP-specific.

3. Moving GLOBAL_SYM, LOCAL_SYM, SESSION_SYM from
   keyword_sp_var_and_label to a separate rule keyword_sysvar_type.
   We don't have system variables with these names anyway.
   Adding ident_sysvar_name and using it in the grammar that needs
   a system variable name instead of ident_or_text.
   This removed a number of shift/reduce conflicts
   between GLOBAL_SYM/LOCAL_SYM/SESSION_SYM as a variable scope and
   as a variable name.

4. Moving keywords BEGIN_SYM, END (in both *.yy fiels)
   and EXCEPTION_SYM (in sql_yacc_ora.yy) into a separate
   rule keyword_sp_block_section, because in Oracle verb keywords
   (COMMIT, DO, HANDLER, OPEN, REPAIR, ROLLBACK, SAVEPOINT, SHUTDOWN, TRUNCATE)
   are good variables names and can appear in e.g. DECLARE,
   while block keywords (BEGIN, END, EXCEPTION) are not good variable names
   and cannot appear in DECLARE.

5. Further splitting keyword_directly_not_assignable in sql_yacc_ora.yy:
   moving keyword_sp_verb_clause out. Renaming the rest of
   keyword_directly_not_assignable to keyword_sp_head,
   which represents keywords that can appear in optional
   clauses in CREATE PROCEDURE/FUNCTION/TRIGGER.

6. Renaming keyword_sp_verb_clause to keyword_verb_clause,
   as now it does not contains anything SP-specific.

   As a result or #4,#5,#6, the rule keyword_directly_not_assignable
   was replaced to three separate rules:
   - keyword_sp_block
   - keyword_sp_head
   - keyword_verb_clause
   Adding the same rules in sql_yacc.yy, for unification.

6. Adding keyword_sp_head and keyword_verb_clause into keyword_sp_decl.
   This fixes MDEV-16244.

7. Reorganizing the rest of keyword related rules into two groups:
  a. Rules defining a list of keywords and consisting of only terminal symbols:
    - keyword_sp_var_not_label
    - keyword_sp_head
    - keyword_sp_verb_clause
    - keyword_sp_block_section
    - keyword_sysvar_type

  b. Rules that combine the above lists into keyword places:
    - keyword_table_alias
    - keyword_ident
    - keyword_label
    - keyword_sysvar_name
    - keyword_sp_decl
  Rules from the group "b" use on the right side only rules
  from the group "a" (with optional terminal symbols added).
  Rules from the group "b" DO NOT mutually use each other any more.
  This makes them easier to read (and see the difference between them).

  Sorting the right sides of the group "b" keyword rules alphabetically,
  for yet better readability.

sql/sql_lex.h

@@ -3630,6 +3630,17 @@ public:
   Item *make_item_colon_ident_ident(THD *thd,
                                     const Lex_ident_cli_st *a,
                                     const Lex_ident_cli_st *b);
+  // For "SELECT @@var", "SELECT @@var.field"
+  Item *make_item_sysvar(THD *thd,
+                         enum_var_type type,
+                         const LEX_CSTRING *name)
+  {
+    return make_item_sysvar(thd, type, name, &null_clex_str);
+  }
+  Item *make_item_sysvar(THD *thd,
+                         enum_var_type type,
+                         const LEX_CSTRING *name,
+                         const LEX_CSTRING *component);
   void sp_block_init(THD *thd, const LEX_CSTRING *label);
   void sp_block_init(THD *thd)
   {