Add function mysql_create_routine()

There is a huge piece of code in mysql_execute_command():

case SQLCOM_CREATE_PROCEDURE:

case SQLCOM_CREATE_SPFUNCTION:

{

  // 120 lines of code

}

This code will be needed for MDEV-10591, to create package routines.

We'll move this code from mysql_execute_command() into a separate new function mysql_create_routine().

This change was originally a part of MDEV-11952, but it was decided to commit it as a standalone change.