(sigframe::~sigframe): Use unregister method.
(sigframe::call_signal_handler): Declare new method.
* exceptions.cc (sigframe::call_signal_handler): New method. Unregisters
current sigframe before calling signal handler.
(setup_handler): Clear waiting threads prior to arming signal_arrived.
* syscalls.cc (_read): Change goto to loop. Recalculate sigframe inside of
loop so that constructor is called when appropriate.
* wait.cc (wait4): Ditto.
* signal.cc: Change "sig" to "signal" in debugging messages throughout.
* sigproc.cc: Ditto.
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