Privilege dropping dosn't load full list of user groups

My setup uses a local user that is inside the docker group to allow access to docker; when now running localhostd with the -u option, it complains about not being able to read /var/run/docker.sock; propably bc the code for changing the user only considers the uid and the primary gid.

Maybe localhostd can draw inspiration from gosu for this: https://github.com/tianon/gosu/blob/master/setup-user.go

(Also: Hii :3)