use constant for ip kind (#151)

IPV6_MULTICAST_HOPS already declared in posix.nim while IP_MULTICAST_TTL
has a value that will stay constant on linux/x86_64 - better codegen and
nlvm compat like this.

chronos/transports/datagram.nim

@@ -15,10 +15,11 @@ when defined(windows):
   import winlean
 else:
   import posix
+  when (defined(linux) and not defined(android)) and defined(amd64):
+    const IP_MULTICAST_TTL: cint = 33
+  else:
     var IP_MULTICAST_TTL* {.importc: "IP_MULTICAST_TTL",
                             header: "<netinet/in.h>".}: cint
-  var IPV6_MULTICAST_HOPS* {.importc: "IPV6_MULTICAST_HOPS",
-                             header: "<netinet/in.h>".}: cint
 
 type
   VectorKind = enum