Modify pages_map() to support mapping uncommitted virtual memory.

If the OS overcommits:
- Commit all mappings in pages_map() regardless of whether the caller
  requested committed memory.
- Linux-specific: Specify MAP_NORESERVE to avoid
  unfortunate interactions with heuristic overcommit mode during
  fork(2).

This resolves #193.

src/jemalloc.c

@@ -1272,6 +1272,7 @@ malloc_init_hard_a0_locked(tsd_t *tsd)
 				abort();
 		}
 	}
+	pages_boot();
 	if (base_boot())
 		return (true);
 	if (chunk_boot())