MDEV-10581 sql_mode=ORACLE: Explicit cursor FOR LOOP
  MDEV-12098 sql_mode=ORACLE: Implicit cursor FOR loop

Cleanup changes:
- Removing sp_lex_cursor::m_cursor_name
- Adding sp_instr_cursor_copy_struct::m_cursor (the cursor global index)
- Fixing sp_instr_cursor_copy_struct::print() to access to the cursor
  name using m_ctx and m_cursor (like other cursor related instructions do)
  instead of m_cursor_name.

This change is needed to unify sp_assignment_lex and sp_cursor_lex later,
to fix this problem easier:
 MDEV-16558 Parenthesized expression does not work as a lower FOR loop bound