Add pool state /proc entry, "SUSPENDED" pools

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

include/libzfs.h

@@ -301,6 +301,8 @@ int zfs_dev_is_whole_disk(char *dev_name);
 char *zfs_get_underlying_path(char *dev_name);
 char *zfs_get_enclosure_sysfs_path(char *dev_name);
 
+const char *zpool_get_state_str(zpool_handle_t *);
+
 /*
  * Functions to manage pool properties
  */