PR27859: correct 404-latch bug in debuginfod client reuse

PR27859: correct 404-latch bug in debuginfod client reuse

PR27701 implemented curl handle reuse in debuginfod_client objects,
but with an unexpected bug.  Server responses returning an error
"latched" because the curl_easy handles for error cases weren't all
systematically removed from the curl multi handle.  This prevented
their proper re-addition the next time.

This version of the code simplfies matters by making only the curl
curl_multi handle long-lived.  This turns out to be enough, because it
can maintain a pool of long-lived http/https connections and related
data, and lend them out to short-lived curl_easy handles.  This mode
handles errors or hung downloads even better, because the easy handles
don't undergo complex state transitions between reuse.

A new test case confirms this correction via the federating debuginfod
instance (cleaning caches between subtests to make sure http* is being
used and reused).

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>