Add "zpool nagios" command

This command is mostly for my personal interest. When executed as such zpool
will behave like a Nagios plugin - outputs a single line of information
about the health of the system for at-a-glance information and exits
with a code of 0=OK, 1=WARNING, 2=CRITICAL, 3=UNKNOWN

Parameters of -o, -u and -c for (referencing the return codes of
OK, Warning, Critical) tweak the return codes to be more or less strict
of minor mishaps.