Sending a 403 to dirs with no index file requests

[douxxtech]

To follow most web servers logic, we now send a 403 when a directory with no index file is requested, instead of the previous 404.

[github-actions]

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.93ms    8.91ms 213.97ms   99.48%
    Req/Sec     1.70k   352.05     2.48k    67.25%
  135358 requests in 20.04s, 207.19MB read
  Socket errors: connect 0, read 426, write 0, timeout 0
Requests/sec:   6755.03
Transfer/sec:     10.34MB