Finalize initial TFTP support

- Finalize TFTP engine, basic download now works and is
  integrated fully with the rest of uftpd
- Remove hard coded TFTP port
- New command line parameters, incompatible change!
- Clarify copyright claims, there are only a few remnants left of the
  FtpServer code by Xu Wang, most of it is refactored now
- Remove string.c, simplify the necessary code in handle_PASV() instead
- General code cleanup

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>