Multiple improvements to selectors.

* Added ``getFd`` procedure for retrieving the underlying
  selector's FD.
* Selectors module's procedures now accept an ``int`` as well as a
  ``SocketHandle``.
* ReadyKey now contains the error code for Event.Error events.

lib/pure/ioselects/ioselectors_poll.nim

@@ -141,7 +141,7 @@ template checkFd(s, f) =
   if f >= s.maxFD:
     raiseIOSelectorsError("Maximum number of descriptors is exhausted!")
 
-proc registerHandle*[T](s: Selector[T], fd: SocketHandle,
+proc registerHandle*[T](s: Selector[T], fd: int | SocketHandle,
                         events: set[Event], data: T) =
   var fdi = int(fd)
   s.checkFd(fdi)
@@ -149,7 +149,7 @@ proc registerHandle*[T](s: Selector[T], fd: SocketHandle,
   setKey(s, fdi, events, 0, data)
   if events != {}: s.pollAdd(fdi.cint, events)
 
-proc updateHandle*[T](s: Selector[T], fd: SocketHandle,
+proc updateHandle*[T](s: Selector[T], fd: int | SocketHandle,
                       events: set[Event]) =
   let maskEvents = {Event.Timer, Event.Signal, Event.Process, Event.Vnode,
                     Event.User, Event.Oneshot, Event.Error}
@@ -280,7 +280,7 @@ template isEmpty*[T](s: Selector[T]): bool =
   (s.count == 0)
 
 proc contains*[T](s: Selector[T], fd: SocketHandle|int): bool {.inline.} =
-  return s.fds[fd].ident != 0
+  return s.fds[fd.int].ident != 0
 
 proc getData*[T](s: Selector[T], fd: SocketHandle|int): var T =
   let fdi = int(fd)
@@ -314,3 +314,7 @@ template withData*[T](s: Selector[T], fd: SocketHandle|int, value, body1,
     body1
   else:
     body2
+
+
+proc getFd*[T](s: Selector[T]): int =
+  return -1