darwin support

Retry of https://github.com/mackerelio/mackerel-agent/tree/os-darwin. Currently just compiles and supports only minimum spec/metrics.

- Moved `metricLinux.PluginGenerator` to `metric.PluginGenerator` because most code are reusable.
This comment disagrees with the substance. When the retry limit is 60 times, the total wait time becomes 30sec * 60 = 30 min.

This maybe comes from the original comment which is copied from.