Add a request/release mechanism to the kernel (x86 only for now) for NMIs.

This version has a lot of updates, mostly changes for comments from John 
Levon.  I have modified the nmi watchdog to use this interface, and it 
seems to work ok.  Keith Owens is copied to see if he would be 
interested in converting kdb to use this, if it gets put into the kernel.

-Corey