conncache: apply multi limits to transfers using a shared pool

- Determine max host and total connection limits from the transfer's own
  multi handle and test that it works for shared connections.

Prior to this change, since df67269 (precedes 8.13.0),
Curl_cpool_check_limits() took the limits from the multi handle that
owns the pool. A share-owned pool is not owned by any multi, so the
limit check always passed.

Prior to df67269 the limits came from the transfer's own multi handle.

Fixes https://github.com/curl/curl/issues/22265
Closes https://github.com/curl/curl/pull/22266

docs/libcurl/opts/CURLSHOPT_SHARE.md

@@ -79,6 +79,11 @@ in different threads using a shared connection.
 Note that when you use the multi interface, all easy handles added to the same
 multi handle share the connection cache by default without using this option.
 
+Connection limits set with CURLMOPT_MAX_HOST_CONNECTIONS(3) and
+CURLMOPT_MAX_TOTAL_CONNECTIONS(3) also apply to transfers using a shared
+connection cache. Each transfer applies the limits of the multi handle it
+runs on to the shared cache.
+
 ## CURL_LOCK_DATA_PSL
 
 The Public Suffix List stored in the share object is made available to all