let tftpd-hpa change the uid and gid of the process

let tftpd-hpa change the uid and gid of the process

tftpd-hpa try to change the uid and gid of the process before serving a file
and it can't if xinetd start the process with a user other than root.

This result to this following message and tftp can't serve file
 in.tftpd[17609]: cannot set groups for user nobody

To customise user/group of the running process we need to use -u option