Allow accessing server file descriptor

ref #128

Author: jklmnn

src/packet/server/linux/gneiss-packet-server-linux.adb

@@ -0,0 +1,12 @@
+
+package body Gneiss.Packet.Server.Linux with
+   SPARK_Mode
+is
+
+   function Get_Fd (Session : Server_Session) return Integer
+   is
+   begin
+      return Integer (Session.Fd);
+   end Get_Fd;
+
+end Gneiss.Packet.Server.Linux;

src/packet/server/linux/gneiss-packet-server-linux.ads

@@ -0,0 +1,11 @@
+
+generic
+package Gneiss.Packet.Server.Linux with
+   SPARK_Mode
+is
+
+   function Get_Fd (Session : Server_Session) return Integer with
+      Pre  => Initialized (Session),
+      Post => Get_Fd'Result > -1;
+
+end Gneiss.Packet.Server.Linux;