AUTHORS: xbps-repo-checkvers -> xbps-checkvers.
xbps-checkvers: use --longopt=FOO in usage().
xbps-checkvers: added -d,--debug and -R,--repository=url options.

- -d, --debug new option to enable xbps debug output.
- --distdir short option has been changed to -D.
- -R, --repository=url new option to append repos to the xbps list.