Extend the --compatibility flag to cover other defaults

[dralley]

• --compatibility will enable sqlite metadata by default
• --compatibility will leave group metadata uncompressed rather than defaulting to the same compression as everything else

I believe this covers the bases in terms of ensuring createrepo_c 1.0 can still generate repos consumable by ancient RHEL / SUSE without too much effort.

I don't add group_gz (separate gzip-compressed group metadata) back - this is aiming for minimal additional complexity and group_gz simply isn't worth it. It's usually a relatively tiny metadata file anyway.

[dralley]

We got a complaint from a user that was using createrepo_c on their Satellite box and of course the default changes messed with them. They're not really supposed to be doing that, but since Koji and others have had the same issue (needing to support EL7 repos using a createrepo_c package from Fedora), we may as well just make it an easy problem to work around.

It's a simple fix in any case.

[dralley]

@kontura Could you do a quick release w/ these changes?