add getPort to resolve Port(0) (#17559)

* add getPort to resolve Port(0)

* fixup

* use getPort in examples + tests

* address comments: do not re-export Port

testament/lib/stdtest/netutils.nim

@@ -1,6 +1,7 @@
 import std/[nativesockets, asyncdispatch, os]
 
 proc bindAvailablePort*(handle: SocketHandle, port = Port(0)): Port =
+  ## See also `asynchttpserver.getPort`.
   block:
     var name: Sockaddr_in
     name.sin_family = typeof(name.sin_family)(toInt(AF_INET))
@@ -8,5 +9,5 @@ proc bindAvailablePort*(handle: SocketHandle, port = Port(0)): Port =
     name.sin_addr.s_addr = htonl(INADDR_ANY)
     if bindAddr(handle, cast[ptr SockAddr](addr(name)),
                 sizeof(name).Socklen) < 0'i32:
-      raiseOSError(osLastError())
+      raiseOSError(osLastError(), $port)
   result = getLocalAddr(handle, AF_INET)[1]