This patch includes:
- MDEV-19639 sql_mode=ORACLE: Wrong SHOW PROCEDURE output for sysvar:=expr
- MDEV-19640 Wrong SHOW PROCEDURE output for SET GLOBAL sysvar1=expr, sysvar2=expr
- Preparatory refactoring for MySQL WL#4179

Detailed change list:

1. Changing sp_create_assignment_lex() to accept the position
   in the exact query buffer instead of a "bool no_lookahead".
   This actually fixes MDEV-19639.
   In the previous reduction sp_create_assignment_lex() was
   called too late, when the parser went far the from beginning
   of the statement, so only a part of the statement got into
   sp_instr_stmt.

2. Generating "SET" or "SET GLOBAL" inside sp_create_assignment_instr()
   depending on the option type.
   This fixes MDEV-19640.
   In the previous reduction the code passed (through no_lookahead)
   the position of the
   word GLOBAL inside sp_create_assignment_lex(), which
   worked only for the left-most assignment.

3. Fixing the affected rules to use:
   - ident_cli instead of ident
   - ident_cli_set_usual_case instead of ident_set_usual_case

4. Changing the input parameter in:
   - LEX::set_system_variable()
   - LEX::call_statement_start()
   - LEX::set_variable()
   from just LEX_CSTRING to Lex_ident_sys_st for stricter data type constrol:
   to make sure that noone passes an ident_cli
   (a fragment of the original query in the client character set)
   instead of server-side identifier
   (utf8 identifier allocated on THD when needed).

5. Adding Lex_ident_sys() in places where the affected functions are called.

6. Moving all calls of sp_create_assignment_lex() to the places
   just before parsing set_expr_or_default.
   This makes the grammar clearer, because
   sp_create_assignment_lex() and sp_create_assignment_instr()
   now stay near each other, so the balance of LEX's push/pop
   can be read easier.
   This will also help to WL#4179.

7. Adding class sp_lex_set_var
   Moving the initialization code from
   sp_create_assignment_lex() to the constructor of sp_lex_set_var.
   This will also help to WL#4179.

8. Moving a part of the "set" grammar rule into a separate
   rule "set_param".
   This makes the grammar easier to read and removes
   one shift/reduce conflict.