UPnP: cache failed XML fetches + demote routine discovery logs (#622) - #653

References #622.

Bug

libupnp ThreadPoolAdd too many jobs: 100 bursts on busy LANs, even after the NT-based SSDP filter in #623. @Stoatwblr's tcpdump shows ~10-13 SSDP advertisements/sec from 5 emitters; one of them (a Zyxel mesh WAP at 192.168.11.179) announces as upnp:rootdevice (so #623's filter must let it through) but its HTTP server is unreachable, so every UpnpDownloadXmlDoc() call against it pins a libupnp worker thread for the full TCP-connect timeout. Multiple concurrent fetches from a single misbehaving device saturate libupnp's pool of 100 and trigger the warning in bursts of ~20 at long intervals.

Two separate fixes in this PR, both targeting the same issue.

1. Per-URL fetch-failure cache (load reduction)

Add a per-CUPnPControlPoint std::map<std::string, time_t> keyed on the location URL of failed UpnpDownloadXmlDoc() calls, guarded by CUPnPMutex. The discovery callback now consults ShouldSkipAdvertisementFetch() before calling UpnpDownloadXmlDoc() on UPNP_DISCOVERY_ADVERTISEMENT_ALIVE events; if the URL has failed within FAILED_FETCH_TTL_SECS (300 s) the announcement is dropped at the callback entry with no HTTP attempt. On successful fetch the entry is evicted; on failure the timestamp is refreshed.

UPNP_DISCOVERY_SEARCH_RESULT events bypass the cache entirely — those are active polls initiated by amule's own search timer and should attempt the fetch regardless. Only the passive ALIVE bursts (where the device drives the rate) get throttled.

The cache stays tiny in practice (handful of misbehaving devices per LAN) and self-prunes via the TTL expiry path on the next post-TTL fetch attempt.

2. Demote two routine log lines (noise reduction)

Two AddDebugLogLineC sites fire once per service-discovery cycle on every successful WAN-service detection — the "WAN Service Detected" line in CUPnPService::CUPnPService (UPnPBase.cpp:590) and the "Successfully retrieved SCPD Document" line in CUPnPControlPoint::Subscribe (UPnPBase.cpp:1736). Both demoted to AddDebugLogLineN — visible under Cat_UPnP=1 but no longer default-on stdout noise on every IGW re-search. #627 demoted other UPnP routine logs but missed these two.

Actionable errors ("WAN service not detected", "Error retrieving device description") stay at AddDebugLogLineC.

Risk surface

• Cache key is the literal location URL string. Different ports on the same host (the Zyxel reporter saw :49152 and :49153) are tracked as separate entries — failures on one don't suppress fetches against the other. Matches least-surprise.
• TTL = 5 minutes. Long enough to absorb a typical SSDP advertisement burst cycle; short enough that genuine intermittent reachability isn't blocked indefinitely.
• SEARCH_RESULT path is untouched, so amule's periodic poll of the network still attempts every URL freshly.
• Side cleanup in the discovery callback: collapsed three inner #if UPNP_VERSION >= 10800 / #else blocks that were duplicating identical format strings differing only by which discovery variable they read from; resolved into a single const char *location at the top of the block. The outer #if UPNP_VERSION shim for actual API differences (struct type, accessor functions) is preserved.

Reported by @Stoatwblr in #622 with tcpdump advertising-rate measurements and a full topology breakdown.