uftpd v2.4

Bug fix release.

Changes

• Handle non-chrooted use-cases better, ensure CWD starts with /
• Increased default inactivty timer: 20 sec --> 180 sec
• Ensure FTP PASV and PORT sockets are set non-blocking to prevent
  blocking the event loop
• README.md updates, add usage section and improve build + install

Fixes

• Fix 100% CPU issue. Triggered sometimes when a user issued CWD ..