curl-util: bring CurlGlue/CurlSlot in line with sd-bus and qmp-client

curl-util: bring CurlGlue/CurlSlot in line with sd-bus and qmp-client

Refactor curl-util to use the same per-request, refcounted, cancellable
slot model as sd-bus, sd-varlink and qmp-client.

CurlGlue becomes opaque and refcounted, and dispatches per-slot
completion callbacks through CURLOPT_PRIVATE instead of a single
g->on_finished demux that every caller had to switch on. The new
curl_glue_perform_async(g, easy, cb, userdata, &slot) replaces
curl_glue_add + the on_finished/userdata wiring.

CurlSlot is the per-request handle: it owns the easy handle,
curl_slot_unref does curl_multi_remove_handle + curl_easy_cleanup
(which doubles as cancel since remove aborts in-flight transfers
without queuing CURLMSG_DONE), and floating slots (ret_slot=NULL) are
kept alive in the glue's slot set until the callback fires. Drop the
userdata parameter from curl_glue_make: CURLOPT_PRIVATE is now used
internally to route completions to the slot.

Migrate pull-job and the pull-{oci,raw,tar} drivers, and imdsd, to the
new shape. PullJob.curl becomes PullJob.slot; pull_job_curl_on_finished
becomes a per-slot callback. imdsd routes its token-vs-data branch off
slot identity rather than easy-handle pointer comparison. Both daemons
drop the global on_finished/userdata wiring on the glue. pull_job_finish
and context_fail{,_full} now return int (always 0) so the callbacks
stay in the `return finish(...);` style.

Add test-curl-util covering glue lifecycle, easy-handle defaults,
floating and non-floating perform paths, cancel-via-slot-unref (verified
by a sentinel request that drives the loop to completion), and three
concurrent requests on a single glue. Tests fetch local files via
file:// URLs so no network is needed; libcurl availability is probed
once via dlopen_curl in intro().