http.c: inline `set_curl_keepalive()`

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.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>