Add pool state /proc entry, "SUSPENDED" pools (SPL)

Add pool state /proc entry, "SUSPENDED" pools (SPL)

(This is the SPL backported code from f0ed6c744)

1. Add a proc entry to display the pool's state:

$ cat /proc/spl/kstat/zfs/tank/state
ONLINE

This is done without using the spa config locks, so it will
never hang.

2. Fix 'zpool status' and 'zpool list -o health' output to print
"SUSPENDED" instead of "ONLINE" for suspended pools.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #7331
Closes #7563