Implement SYSLOG handler support for Audit Log v2 plugin

Hi,

The new feature was verified. The logs are written to syslog. Vars changes are applied as expected.

Made some additional changes in scope of this task - https://github.com/percona/percona-server/pull/4602/commits/7fa8e43a3a4a325cf459f5f62fc0fed92d2c6d62.

In general, plugin's message format changed to the following:

Now audit plugin doesn't set its own syslog ident and uses the one provided by the server. This helps to avoid situations when the audit plugin rewrites ident for server's messages. The audit_log_filter_syslog_ident sysvar is replaced with audit_log_filter_syslog_tag{}, which is used as a prefix for a message (test_audit in example above).