write-out: make %header{} able to output *all* occurances of a header

[bagder]

By appending :all:[separator] to the header name. The [separator] string is output between each header value if there are more than one to output.

Test 764 verifies

Idea-by: kapsiR on github
Ref: #18449

[kapsiR]

Great! Thanks for the quick implementation!

I don't know the curl internals enough, the only thing I'd mention here is, if there is a possibility of a recursion, should there any handling for that? (or is this not possible due to the redirect implementation?)

[bagder]

I don't know the curl internals enough, the only thing I'd mention here is, if there is a possibility of a recursion, should there any handling for that? (or is this not possible due to the redirect implementation?)

That's not a risk here. There are several different ways those are prevented (before this code runs): there's a maximum redirections to follow value (--max-redirs, 50 by default) and there is a maximum number of headers curl will record before it returns error.