files_backend
typedef struct files_backend *files_backend_ptr_t;
netgr_get_members(struct files_backend *be,
top_down(struct files_backend *be, const char **groups, int ngroups,
netgr_set(struct files_backend *be, void *a)
netgr_in(struct files_backend *be, void *a)
netgr_destr(struct files_backend *be, void *dummy)