mmapped heaps: fix the file descriptor argument to mmap for the BSDs

On the BSDs, the file descriptor passed to mmap when using MAP_ANON
must be -1.  Passing 0 causes mmap to fail.

gc.c

@@ -501,7 +501,7 @@ sexp_heap sexp_make_heap (size_t size, size_t max_size, size_t chunk_size) {
   sexp_heap h;
 #if SEXP_USE_MMAP_GC
   h =  mmap(NULL, sexp_heap_pad_size(size), PROT_READ|PROT_WRITE|PROT_EXEC,
-            MAP_ANON|MAP_PRIVATE, 0, 0);
+            MAP_ANON|MAP_PRIVATE, -1, 0);
   if (h == MAP_FAILED) return NULL;
 #else
   h =  sexp_malloc(sexp_heap_pad_size(size));