User-visible:
- syslog/Eventlog writer:
  Variables moved from the server-proper into the sys/eventlog
  component, and be namespaced accordingly:

  @@global.log_syslog_tag         -> @@global.syseventlog.tag
  @@global.log_syslog_facility    -> @@global.syseventlog.facility
  @@global.log_syslog_include_pid -> @@global.syseventlog.include_pid
  @@global.log_syslog             -> removed, use  @@global.log_error_services

Log-service API:
- new call characteristics() tells us whether a service is read-only,
  multi-open, its type, etc.; log_service_chistics replaces
  log_service_builtin_type
- removed calls for interim sysvar handler now that all log-services
  are switched over to component variables
- connection_loop_aborted() removed

Code & tests:
- adds test for Eventlogging (on Windows)  (Bug#28238071)
- added testing of SET PERSIST of syseventlog parameters (Bug#27534089)
- added testing of syseventlog parameters at start-up
- deduplicate syseventlog code in server (by having log_builtins use mysys)
- better diagnostics when bad arguments are provided at start-up
- message that was a string literal moved from code to errmsg file
- --log-syslog=0 removed; mysql-test-run and tests upated accordingly
- updated an error message that referred to removed --log-syslog
- log_builtins_error_stack() comments extended and signature cleaned up