add valgrind.sh

For running station-sniffer (or the test clients) under valgrind
(memcheck)

Signed-off-by: Tucker Polomik <t.polomik@cablelabs.com>

valgrind.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+VALGRIND="valgrind"
+if ! [ -x "$(command -v ${VALGRIND})" ]; then
+  echo "{$VALGRIND} not found. skipping..."
+  exit 1
+fi
+
+valgrind --error-exitcode=1 --tool=memcheck --leak-check=full --track-origins=yes -q --error-limit=no --gen-suppressions=all "$@"