Add "zpool nagios" command

(Though incomplete this feature is working.)

The "zpool nagios" command behaves as a Nagios plugin - output a
single line of status information to stdout and exit with a status
code (0=OK, 1=Warning, 2=Critical, 3=Unknown).

There are a few cases that could use improvement for multi-pool
systems, and not all failure scenarios are tested, but it's
certainly a step in the right direction.

TODO:
* Specify pools which must be present on the system

* Deadman timer of sorts in case ioctls hang