Bug #36098 Audit plugin (wl 3771) feature disabled in 6.0
avoid recusrive locking of LOCK_plugin

include/mysql/plugin_audit.h:
  fix incorrect version
sql/log.cc:
  move the common code to a shared header
sql/mysqld.cc:
  restore the deleted functionality
sql/set_var.cc:
  remove unused parameter
sql/sql_audit.h:
  two inline convenience functions
sql/sql_parse.cc:
  use a simplified convenience call
sql/sql_plugin.cc:
  unlock LOCK_plugin for plugin->init() call, add missing OOM check,
  issue "unknown variable" error in find_sys_var, not down the stack