Re: [PATCH v2 3/4] http.c: inline `set_curl_keepalive()`

> At the end of `get_curl_handle()` we call `set_curl_keepalive()` to
> enable TCP keepalive probes on our CURL handle. `set_curl_keepalive()`
> dates back to 47ce115370 (http: use curl's tcp keepalive if available,
> 2013-10-14), which conditionally compiled different variants of
> `set_curl_keepalive()` depending on what version of curl we were
> compiled with[^1].
> 
> As of f7c094060c (git-curl-compat: remove check for curl 7.25.0,
> 2024-10-23), we no longer conditionally compile `set_curl_keepalive()`
> since we no longer support pre-7.25.0 versions of curl. But the version
> of that function that we kept is really just a thin wrapper around
> setting the TCP_KEEPALIVE option, so there's no reason to keep it in its
> own function.
> 
> Inline the definition of `set_curl_keepalive()` to within
> `get_curl_handle()` so that the setup of our CURL handle is
> self-contained.
> 
> [1]: The details are spelled out in 47ce115370, but the gist is curl
>   7.25.0 and newer use CURLOPT_TCP_KEEPALIVE, older versions use
>   CURLOPT_SOCKOPTFUNCTION with a custom callback, and older versions
>   that predate even that option do nothing.