kernel - Rewrite the callout API, major structural changes

kernel - Rewrite the callout API, major structural changes

* Make struct callout much smaller, primarily now just containing
  a pointer to a shadow structure which is allocated on first use.

  The callout backend uses this shadow structure on the callout
  lists instead of the embedded struct callout.  This should reduce
  occurances of untraceable panics due to list corruption.  Instead,
  the callout-v-shadow structure linkages are validated with an assertion
  and if this panics it should be possible to determine which subsystem
  was responsible from the kernel core.

* tsleep() etc is used early in the boot sequence and use some
  bypass functions to streamline operations and avoid allocation
  calls.  This should also improve tsleep()s performance a bit over
  the original implementation.