*: remove capnslog, link to new home

[jonboulle]

This eliminates a circular dependency between coreos/pkg and
coreos/go-systemd.

See #73 for more details.

[jonboulle]

@lucab wdyt?

[lucab]

This will probably break all of https://godoc.org/github.com/coreos/pkg/capnslog?importers, but I'm not sure if there exists a gentler deprecation plan.

[jonboulle]

I'd say generally speaking the onus is on projects to vendor but I see a couple of ours (like https://github.com/coreos/go-oidc) aren't doing it - so not quite sure at this stage.

[lucab]

go-oidc doesn't depend on this anymore after coreos/go-oidc#95.

[bgilbert]

I've archived the separate capnslog repo and I'm going to close this out. It looks like the existing users are all still importing from coreos/pkg. Due to the light maintenance of capnslog and this repo generally, packages should probably be migrating to a different logging library; let's not add more churn in the meantime.

See also #57.