e.g.MYSQL_AUDIT_GENERAL_ERROR

General audit API (MYSQL_AUDIT_GENERAL_CLASS) didn't expose event
subclass to plugins.

This patch exposes event subclass to plugins via
struct mysql_event_general::event_subclass.

This change is not compatible with existing general audit plugins.
Audit interface major version has been incremented.

include/mysql/plugin_audit.h:
  Expose event subclass to audit general plugins.
plugin/audit_null/audit_null.c:
  Added distinct counters for general event sub-classes.
  
  Removed printf() from deinit(). One can easily see number of
  calls via status variables.
  
  To make code nicer, modified class mask to use macro instead
  of hardcoded number.
  
  Incremented audit plugin minor version.
sql/sql_audit.cc:
  Expose event subclass to audit general plugins.