Use dmalloc instead of EFence when available (dmalloc has lot of improvements

over EFence and also hopefully smaller memory overhead, but sadly it's non-free
for commercial use).

If the DMALLOC_OPTIONS environment variable is not set, switch on `reasonable'
checks by default.

Also introduced mb_allocz() for cleared mb_alloc().

lib/resource.c

@@ -8,6 +8,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "nest/bird.h"
 #include "lib/resource.h"
@@ -156,6 +157,14 @@ mb_alloc(pool *p, unsigned size)
   return b->data;
 }
 
+void *
+mb_allocz(pool *p, unsigned size)
+{
+  void *x = mb_alloc(p, size);
+  bzero(x, size);
+  return x;
+}
+
 void
 mb_free(void *m)
 {