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.nim

@@ -3458,6 +3458,10 @@ when not defined(JS): #and not defined(nimscript):
       ## allows you to override the behaviour of your application when CTRL+C
       ## is pressed. Only one such hook is supported.
 
+    when not defined(useNimRtl):
+      proc unsetControlCHook*()
+        ## reverts a call to setControlCHook
+
     proc writeStackTrace*() {.tags: [], gcsafe.}
       ## writes the current stack trace to ``stderr``. This is only works
       ## for debug builds. Since it's usually used for debugging, this

lib/system/embedded.nim

@@ -40,6 +40,7 @@ proc reraiseException() {.compilerRtl.} =
 
 proc writeStackTrace() = discard
 
+proc unsetControlCHook() = discard
 proc setControlCHook(hook: proc () {.noconv.}) = discard
 
 proc closureIterSetupExc(e: ref Exception) {.compilerproc, inline.} =

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)