Ensure a flavourless page always exists for all flavourful ports

[cnst]

With ports-readmes, some ports don't seem to have any .html pages generated without a flavour always being part of the .html filename.

This is all fine and dandy if you're just following the links within ports-readmes itself, but such approach breaks down if someone wants to manually construct a URL based on the structure of the ports tree filesystem. E.g. we can easily give links to openports.se or CVSweb, but they wouldn't necessarily be able to give correct links back to us, due to the flavours.

For example, there is no /databases/sqlports.html, only /databases/sqlports,-main.html. (ATM, we solve this specific case on ports.su through an if-file-exists and a 302 redirect in the nginx.conf, but the approach is not universal.)

There should always be a page generated for every port that does not include flavours in its filename. Such page could use the category.tt2 template, or, potentially be a 300 Multiple Choices or even a 302 Found, and provide proper links to existing flavourful pages of a given port. It would only exist if its filename has not already been taken by a regular ports page.