Cherry-pick #8973 to 6.5: Minor Journalbeat fixes and additions

[kvch]

Cherry-pick of PR #8973 to 6.5 branch. Original message:

Refactoring of option seek

Previously, the option was string, so deciding which seeker function has to be called used string comparisons. I added SeekMode so the mode can be an iota and provided its own Unpack function. This also takes care of validating the user configured value.

Field renaming

I renamed custom.* prefix to journald.custom.*, so users know where those custom fields are coming from.

Dashboard

It is a minimal dashboard with a few predefined searches. When modules are available to parse messages coming from journald, it is going to be possible create prettier visualizations.

Skip last event when seek is set to tail

Previously, if seek was set to tail, the last event in the journal was read. Now this last event is skipped to avoid duplication.

Unstoppable Journalbeat (haha)

If the output was unreachable Journalbeat got stuck when it retried to connect to the output. As the Beat never stops trying, it never returned from the last client.Publish call. Thus, publishAll function never stopped, because it never received any signal from the done channel of the input.
The client of each input is closed during Stop of each input.

Registry file path

Previously, Journalbeat put its registry file under /registry when installed from deb package. From now the registry file resides under the folder specified by -path.data.

[jsoriano]

@urso I think we should backport the fix for the registry file for 6.6.0 even if it is breaking, if we release a journalbeat docker as of elastic/beats-docker#58 or #9433 and as some people is already requesting, keeping the registry between docker restarts will require additional steps that would need to be documented. I think it'd be better to fix this while it is experimental.