curl.h: name all public function parameters

[ryandesign]

Most public function parameters already have names; this adds those that were missing.

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I took the parameter names from where the functions were defined in the C sources. In some cases this does not match the documentation. For example, in lib/slist.c (and, with this PR, in curl.h) the second parameter of curl_slist_append is called data while in the documentation it's called string.

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My motivation for making these changes arose when I wrote a script to parse the curl headers in part to get the public function parameter names and I noticed a few functions didn't have them. Do you have any objections to adding these?

[bagder]

Thanks!