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:
  1. 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
  2. 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.