Route fluentd internal log events to specified label

[tagomoris]

Currently, fluentd's internal logs can be captured by using <match fluent.*> sections, but it can't be routed to labels.
I think fluentd should be able to do it, to standardize routing by labels.

[tagomoris]

It's better to suppress log forwarding into plugins before #start completion, and just before calling #stop.
Plugins can process events between #start and #stop. Otherwise, plugins cannot process events in general.

So we should stop emitting Fluentd's log events out of these state, and also should add an option to emit events forcedly always.

[tagomoris]

#485 relates with this.

[tagomoris]

#1405 fixes this issue.