Add parse_errorlog_arg callback to ap_errorlog_provider to allow providers to check the ErrorLog argument. Implement this check in mod_syslog.

Error log providers need to be able to trigger a startup error from their init() function. A NULL return code is the trigger.

Add AP_ERRORLOG_PROVIDER_ADD_EOL_STR flag for ap_errorlog_provider, bump MMN.

allow the provider to decide whether or not APLOG_NOTICE messages should be logged

mod_syslog: New module implementing syslog ap_error_log provider. Previously, this code was part of core, now it's in separate module.