Further 64bit signature fixes

Author: atoker

src/Unix.cs

@@ -225,10 +225,10 @@ unsafe class UnixSocket
 		//static extern int vmsplice (int fd, IOVector* iov, uint nr_segs, uint flags);
 
 		[DllImport ("libc", SetLastError=true)]
-		public static extern int recvmsg (int s, IntPtr msg, int flags);
+		public static extern SSizeT recvmsg (int s, IntPtr msg, int flags);
 
 		[DllImport ("libc", SetLastError=true)]
-		public static extern int sendmsg (int s, IntPtr msg, int flags);
+		public static extern SSizeT sendmsg (int s, IntPtr msg, int flags);
 
 		public int Handle;
 		bool ownsHandle = false;

src/UnixNativeTransport.cs

@@ -66,7 +66,7 @@ unsafe void WriteBsdCred ()
 			cm.hdr.cmsg_level = 0xffff; //SOL_SOCKET
 			cm.hdr.cmsg_type = 0x03; //SCM_CREDS
 
-			int written = UnixSocket.sendmsg (socket.Handle, (IntPtr)(&msg), 0);
+			int written = (int)UnixSocket.sendmsg (socket.Handle, (IntPtr)(&msg), 0);
 			if (written < 0)
 				throw UnixError.GetLastUnixException ();