MDEV-6567: Raw debug output in the error log.

Removed raw debug output when no error on OS operation has not happened.
Basic test of slave_run_triggers_for_rbr variable added.
MDEV-6575: main.view --ps-protocol fails in ANALYZE code

- After PREPARE is done, save the value of thd->select_number
  (this is what will be assigned to next select_lex object)
- Restore the value at each execution of the prepared statement.
Merge branch '10.1' of github.com:MariaDB/server into 10.1
git: ignore sql/share/errmsg.sys, better diff header for C/C++ files
cleanup

* make Item_param::get_settable_routine_parameter() public, because it's
  public in the parent Item class and all other Item descendants too
* split my_var class in three - base my_var and two descencants, move
  properties into descendants, remove if(), create a virtual method instead
* factor out the common code in the select_var_ident parser rule
* preserve the Item_param type of the param_marker in the parser,
  don't upcast cast it to Item
MDEV-5317 out parameters in PREPARE "SELECT ... INTO"

based on the contribution of Antony T Curtis