New utility function xmemdup().

include/util.h

@@ -80,6 +80,7 @@ void set_program_name(const char *);
 void *xmalloc(size_t);
 void *xcalloc(size_t, size_t);
 void *xrealloc(void *, size_t);
+void *xmemdup(const void *, size_t);
 char *xstrdup(const char *);
 char *xasprintf(const char *format, ...) PRINTF_FORMAT(1, 2);