Add zpool state for zfs plugin on Linux

## Feature Request

The state of the zpool is available in /proc/spl/kstat/zfs//state. It would be great to read this state and send it over to InfluxDB.

Example:
```
# cat /proc/spl/kstat/zfs/rpool/state
ONLINE
```

### Use case:

Would like to set alerts in InfluxDB when a zpool gets degraded

Start at the Telegraf zfs plugin entry point and inspect how it reads /proc/spl/kstat/zfs/. Add collection of the pool's state for sending to InfluxDB, using the ONLINE example as a reference. Done means the zpool state is exposed by the plugin and degraded pools can be identified for alerts.

Written by the indexing model from the issue text.