hammer2 - Cleanup error paths
* Do not try to return I/O error status from hammer2_chain_lock() or
hammer2_cluster_lock(). This is just asking for trouble by depending
on callers to keep track of state in local variables.
Instead, accumulate error status in chain->error and cluster->error.
Adjust cluster->error based on focus. Code does not yet check this field.
* Do not NULL-out cluster array entries on I/O errors or if unavailable.
Leave the chain structure intact.
This makes the cluster's array-of-chains more deterministic and will
allow upcoming code to use errored chains as a placeholder for blocking
operations.
* When locking a cluster, attempt to select alternative masters or slaves
for our focus if the chain we would have otherwised used is errored.
* Add inlines to test whether a chain or cluster can be read or written
(writing still also requires calling *_modify()).
* Document leaving inode->pfs_nmasters set to 0 for new masters.
* Adjust hammer2 pfs-list to display whether a master is a normal
master or a snapshot.