kernel - Avoid p->p_token when sending a signal to a particular lwp

kernel - Avoid p->p_token when sending a signal to a particular lwp

* Avoid holding the per-process token excluysively when sending a signal
  to a particular lwp.  Hold it shared to locate the lwp, but otherwise do
  all signal processing with the target lwp's token.

* Significantly improves vkernel timer operation.  Timer distribution to
  multiple cpus no longer trip over themselves.