kernel - Fix additional races in lwp_signotify()

kernel - Fix additional races in lwp_signotify()

* lwp_signotify() was improperly scheduling threads whos td_gd is on the
  local cpu without checking the SINTR flags.  This can catch a thread in
  the middle of being transitioned to another cpu and cause havoc.

* Only schedule the thread if the SINTR flags are set.

* We can't call setrunnable() from an IPI so adjustments have to be made
  in the remote cpu to set the lp's lwp_stat state before issuing the IPI
  and only do the scheduling of its thread from the IPI function.

Reported-by: ftigeot