hammer2 - Support multiple PFS mounts from the same device
* Implement hammer2_pfsmount_t and move hmp->rchain, hmp->iroot, and
a few other fields into the new structure.
struct mount -> struct hammer2_pfsmount (pmp) -> struct hammer2_mount (hmp).
* Implement mount/umount logic to allow the device to be overloaded.
Multiple PFS labels associated with the same media may now be mounted.
* Generate a unique pfs_fsid for each PFS created, the {pfs_fsid, pfs_id}
tuple forms a unique key that the userland helper will use to distinguish
registrations.