S9 EXT  (sys:group-name string|integer)  ==>  string
        (sys:group-gid string|integer)   ==>  integer

Return the group name or GID associated with the group name
STRING or the GID INTEGER. (Yes, getting the group name of a
group name or the GID of a GID is an identity operation.)

(sys:group-name 0)       ==>  "wheel"
(sys:group-gid "wheel")  ==>  0
