fsck.zfs: implement 4/8 exit codes as suggested in manpage

Update the fsck.zfs helper to bubble up some already-known-about 
errors if they are detected in the pool.

health=degraded => 4/"Filesystem errors left uncorrected"
health=faulted && dataset in /etc/fstab => 8/"Operational error"
pool not found => 8/"Operational error"
everything else => 0

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11806

cmd/fsck_zfs/.gitignore

@@ -0,0 +1 @@
+/fsck.zfs