Use kern_sigprocmask() instead of direct manipulation of td_sigmask to
reschedule newly blocked signals.
MFC r198590:
Trapsignal() calls kern_sigprocmask() when delivering catched signal
with proc lock held.
MFC r198670:
For trapsignal() and postsig(), kern_sigprocmask() is called with
both process lock and curproc->p_sigacts->ps_mtx locked. Prevent lock
recursion on ps_mtx in reschedule_signals().
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