HAMMER 65/Many: PFS cleanups and confusion removal

HAMMER 65/Many: PFS cleanups and confusion removal

* Simplify master/slave operation.  Remove the master=, slave, and no-mirror
  options from pfs-update.  Retain pfs-master, pfs-slave, pfs-upgrade, and
  pfs-downgrade for dealing with master and slave operation.

  Move the master= and nomirror specifications back to hammer_mount.  They
  have to be mount-wide anyhow, the current code does not support different
  master ID's on a PFS-by-PFS basis.

* Add some additional failsafes to the mount code.   The master id cannot
  be allowed to change by a mount update (-u).

* Fix a bug in hammer pfs-upgrade operation that could result in a livelock
  during the B-Tree iteration.