overwrite symbol link

```
lrwxrwxrwx. 1 root root 20 Oct 17 13:17 /etc/mock/default.cfg -> fedora-21-x86_64.cfg
lrwxrwxrwx. 1 root root 25 Nov 17 22:23 /etc/mock/default.cfg.rpmnew -> fedora-rawhide-x86_64.cfg

Traceback (most recent call last):
  File "/sbin/rpmconf", line 318, in <module>
    main()
  File "/sbin/rpmconf", line 311, in main
    handle_package(args, package_hdr)
  File "/sbin/rpmconf", line 217, in handle_package
    handle_rpmnew(args, conf_file, conf_file + ".rpmnew")
  File "/sbin/rpmconf", line 159, in handle_rpmnew
    overwrite(args, other_file, conf_file)
  File "/sbin/rpmconf", line 43, in overwrite
    copy(src, dst)
  File "/sbin/rpmconf", line 35, in copy
    os.symlink(linkto, dst)
FileExistsError: [Errno 17] File exists: 'fedora-rawhide-x86_64.cfg' -> '/etc/mock/default.cfg'
```

Reference: https://github.com/xsuchy/rpmconf/issues/11
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>