CONTRIB: trace: report the base name only for file names

The output is hard to read when the full path to the sources files is
emitted, let's strip it.
diff --git a/contrib/trace/trace.awk b/contrib/trace/trace.awk
index 74d953c..7b3b131 100755
--- a/contrib/trace/trace.awk
+++ b/contrib/trace/trace.awk
@@ -37,7 +37,7 @@
   if (prog == "")
     prog=ENVIRON["PROG"];
 
-  cmd=cmd " -f -e " prog;
+  cmd=cmd " -f -s -e " prog;
 
   for (i = 1; i < 100; i++) {
     indents[">",i] = indents[">",i-1] "->"