Include "chown-core.h".

Include "chown-core.h".
[enum Change_status, enum Verbosity]: Remove declarations.
Remove decls of globals that are now part of struct Chown_option.
Remove decl of xstat.
(describe_change): Remove function.
(change_file_group): Likewise.
(change_dir_group): Likewise.
(parse_group): Don't set global, groupname, here...
(main): ... instead, initialize `chopt.group_name' here.
Initialize chopt and update uses of the now-members.
Set group_name also when it's obtained via a --reference=FILE option.
Call change_file_owner (with -1 for uids), not change_file_group.