remote-curl: unbreak http.extraHeader with custom allocators

remote-curl: unbreak http.extraHeader with custom allocators

In 93b980e58f5 (http: use xmalloc with cURL, 2019-08-15), we started to
ask cURL to use `xmalloc()`, and if compiled with nedmalloc, that means
implicitly a different allocator than the system one.

Which means that all of cURL's allocations and releases now _need_ to
use that allocator.

However, the `http_options()` function used `slist_append()` to add any
configured extra HTTP header(s) _before_ asking cURL to use `xmalloc()`,
and `http_cleanup()` would release them _afterwards_, i.e. in the
presence of custom allocators, cURL would attempt to use the wrong
allocator to release the memory.

A naïve attempt at fixing this would move the call to
`curl_global_init()` _before_ the config is parsed (i.e. before that
call to `slist_append()`).

However, that does not work, as we _also_ parse the config setting
`http.sslbackend` and if found, call `curl_global_sslset()` which *must*
be called before `curl_global_init()`, for details see:
https://curl.haxx.se/libcurl/c/curl_global_sslset.html

So let's instead make the config parsing entirely independent from
cURL's data structures. Incidentally, this deletes two more lines than
it introduces, which is nice.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>