make --url support a file with URLs

Hey,

Over the years, people have requested an easier way to provide a list of URLs 
to curl and have it download them all.

With my new PR [*], you can write "curl --url @file" and curl will download 
all the URLs in the provided file as if -O was used for each one of them. It 
can also get the list from stdin if you do "--url @-" in style with how other 
curl options work.

If you add -Z, it will do them in parallel.

Thoughts? What did I forget?

[*] = https://github.com/curl/curl/pull/16099

-- 

  / daniel.haxx.se || https://rock-solid.curl.dev
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