Finally extract the log library and make it build.

This patch:
  - introduces an fdio module for low-level fd functions that don't
    need to log.
  - moves the responsibility for opening files outside of torlog.c,
    so it won't need to call tor_open_cloexec.

src/test/test_util.c

@@ -19,6 +19,7 @@
 #include "common/util_process.h"
 #include "test/log_test_helpers.h"
 #include "lib/compress/compress_zstd.h"
+#include "lib/fdio/fdio.h"
 
 #ifdef HAVE_PWD_H
 #include <pwd.h>