- set_user_and_group(+User) is det
- set_user_and_group(+User, +Group) is det
- Set the UID and GID to the User. User is either a UID or a user
name. If Group is not specified, the primary group of User is
used. If initgroups/2 is available, the resulting group access
list of the calling process consists of the registered groups
for User and the specified Group.