Refactor sp_add_instr_fetch_cursor to get the target list argument

Under terms of MDEV-10152 we'll need LEX::sp_add_instr_fetch_cursor() to get the target list as a parameter, to be able to rewrite it correctly (to the data type of the REF CURSOR .. RETURN clause, instead of setting it post-factum with help of the method sp_instr_fetch_cursor::set_fetch_target_list().

Proposal:

• Change the result type of sp_add_instr_fetch_cursor() from sp_instr_fetch_cursor to bool
• Rename sp_add_instr_fetch_cursor() to sp_add_fetch_cursor() to avoid problems during merge (the opposite return value in bool context)
• Add a new parameter to sp_add_fetch_cursor(), so it looks like this:

  bool sp_add_fetch_cursor(THD *thd,

  const Lex_ident_sys_st &name,

  const List<sp_fetch_target> &list);

• Add a new target list parameter to constructors sp_instr_fetch_cursor, sp_instr_cfetch, sp_instr_cfetch_by_ref, remove the methods sp_instr_fetch_cursor::set_fetch_target_list(), sp_instr_fetch_cursor::add_to_fetch_target_list(), as they won't be needed any more.
• Remove from sql_yacc.yy the grammar rule sp_proc_stmt_fetch_head and add instead a new rule

  fetch_statement_source:

  ident { $$= $1; }

  | FROM ident { $$= $2; }

  | NEXT_SYM FROM ident { $$= $3; }

  ;

• Change the grammar rule sp_proc_stmt_fetch to the following:

  sp_proc_stmt_fetch:

  FETCH_SYM fetch_statement_source INTO sp_fetch_list

  {

  if (unlikely(Lex->sp_add_fetch_cursor(thd, $2, *$4)))

  MYSQL_YYABORT;

  }