MEGA Fixes / Cleanup

--------------------

- Seal the fate of long standing memory leak (4 years, 7 months) during
  pcm_unregister(). While destroying cdevs, scan / detect possible
  children and free its SLIST placeholder properly.
- Optimize channel allocation / numbering even further. Do brute cyclic
  checking only if the channel numbering screwed.
- Mega vchan create/destroy cleanup:
  o Implement pcm_setvchans() so everybody can use it freely instead
    of implementing their own, be it through sysctl or channel auto
    allocation.
  o Increase vchan creation/destruction resiliency:
    + it's possible to increase/decrease total vchans even during
      busy playback/recording. Busy channel will be left alone, untouched.
      Abusive test sample:
      # play whatever...
      #
         while : ; do
           sysctl hw.snd.pcm0.vchans=1
           sysctl hw.snd.pcm0.vchans=10
           sysctl hw.snd.pcm0.vchans=100
           sysctl hw.snd.pcm0.vchans=200
         done
      # Play something else, leave above loop running frantically.
    + Seal another 4 years old bug where it is possible to destroy (virtual)
      channel even when its cdevs being referenced by other process.
      The "First Come First Served" nature of dsp_clone() is the main
      culprit of this issue, and usually manifest itself as dangling
      channel <-> process association. Ensure that all of its cdevs
      are free from being referenced before destroying it (through
      ORPHAN_CDEVT() macross).

All these fixes (including previous fixes) will be MFCed, later.

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