kernel - Major signal path adjustments to fix races, tsleep race fixes, +more

kernel - Major signal path adjustments to fix races, tsleep race fixes, +more

* Refactor the signal code to properly hold the lp->lwp_token.  In
  particular the ksignal() and lwp_signotify() paths.

* The tsleep() path must also hold lp->lwp_token to properly handle
  lp->lwp_stat states and interlocks.

* Refactor the timeout code in tsleep() to ensure that endtsleep() is only
  called from the proper context, and fix races between endtsleep() and
  lwkt_switch().

* Rename proc->p_flag to proc->p_flags

* Rename lwp->lwp_flag to lwp->lwp_flags

* Add lwp->lwp_mpflags and move flags which require atomic ops (are adjusted
  when not the current thread) to the new field.

* Add td->td_mpflags and move flags which require atomic ops (are adjusted
  when not the current thread) to the new field.

* Add some freeze testing code to the x86-64 trap code (default disabled).