Add some utilities to make it easier to use libcurl URL feature flags

[rdw-software]

It's possible to set the feature flags for curl.url_set using bit operations, but that's probably not the most convenient way to do it. Because curl uses a bitmask, named enum values won't work. I'm also not sure how important it is to modify the flags conveniently. For now, I guess this can wait.

See https://curl.se/libcurl/c/curl_url_set.html

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Split off from #653.

[rdw-software]

Before implementing anything, it would be useful to see which other libraries require using a similar bitmask-style API.
Ideally, a general solution could be devised that allows setting, unsetting, and querying features a bit more easily.