add unsetControlCHook to remove a Ctrl-C hook after it was set (#7267)

* add unsetControlCHook to remove a Ctrl-C hook after it was set

Adds the inverse proc to setControlCHook in order to lift a Ctrl-C
hook after it has been set.

* remove check for noSignalHandler in system/excpt.nim

lib/system/excpt.nim

@@ -536,3 +536,8 @@ proc setControlCHook(hook: proc () {.noconv.}) =
   # ugly cast, but should work on all architectures:
   type SignalHandler = proc (sign: cint) {.noconv, benign.}
   c_signal(SIGINT, cast[SignalHandler](hook))
+
+when not defined(useNimRtl):
+  proc unsetControlCHook() =
+    # proc to unset a hook set by setControlCHook
+    c_signal(SIGINT, signalHandler)