Add os/sigaction for signal handling.

Also improve interrupts to work better with busy loops
and signals.

examples/sigaction.janet

@@ -0,0 +1,10 @@
+(defn action []
+  (print "Handled SIGHUP!")
+  (flush))
+
+(defn main [_]
+  # Set the interrupt-interpreter argument to `true` to allow
+  # interrupting the busy loop `(forever)`. By default, will not
+  # interrupt the interpreter.
+  (os/sigaction :hup action true)
+  (forever))