hammer2 - more dmsg/separation work

* More separation work in libdmsg, kern_dmsg.c, and kernel dmsg consumers.

* subr_diskiocom.c should no longer need hammer2_disk.h (sys/dmsg.h)
  for the kernel build.

* Add dmsg->tcmd to smooth some switch() use cases.
hammer2 - more dmsg/separation work

* Use msg->tcmd in a few places.

* Remove some debugging.

* Move the iocom in the hammer2 VFS from the pmp to the hmp, allowing
  all supported PFSs to be advertised without having to mount each one.

* Preliminary work on hammer2-generated SPANs for all PFSs on a
  hammer2_volume.  Since multiple PFSs can be advertised, we can
  no longer use AUTOTXSPAN (which can only transmit one).

* More adjustments to newfs_hammer2.  When generating multiple PFSs,
  each one is given its own clid and fsid.