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