Use shorter labels; using long labels creates problems on the Macintosh.

(Thanks, Just!)
diff --git a/Doc/lib/libxdrlib.tex b/Doc/lib/libxdrlib.tex
index 6157c9d..56459f8 100644
--- a/Doc/lib/libxdrlib.tex
+++ b/Doc/lib/libxdrlib.tex
@@ -206,7 +206,7 @@
 
 
 \subsection{Exceptions}
-\nodename{Exceptions in xdrlib module}
+\nodename{xdr-exceptions}
 
 Exceptions in this module are coded as class instances: