Implement and use an xstrdup() function

Many places in dtc use strdup(), but none of them actually check the
return value to see if the implied allocation succeeded.  This is a
potential bug, which we fix in the patch below by replacing strdup()
with an xstrdup() which in analogy to xmalloc() will quit with a fatal
error if the allocation fails.

I felt the introduciton of util.[ch] was a better choice
for utility oriented code than directly using srcpos.c
for the new string function.

This patch is a re-factoring of Dave Gibson's similar patch.

Signed-off-by: Jon Loeliger <jdl@freescale.com>

util.c

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2008 Jon Loeliger, Freescale Semiconductor, Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program; if not, write to the Free Software
+ *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
+ *                                                                   USA
+ */
+
+#include "dtc.h"
+
+char *xstrdup(const char *s)
+{
+	int len = strlen(s) + 1;
+	char *dup = xmalloc(len);
+
+	memcpy(dup, s, len);
+
+	return dup;
+}