known2.met: O(1) AICH SaveHashSet dedup (fixes #579) - #581

known2_64.met is the AICH (Advanced Intelligent Corruption Handling)
hashset cache: a Merkle tree of SHA-1 hashes per shared file, used
by peers to recover from sub-part-granularity corruption. Each entry
is keyed by its AICH root hash; ngosang's #597 report had it at
1.1 GB. The file has no mtime-touch bloat (entries are
content-addressed, dedup is already in place since #581), but it
never shrinks: once a hashset is cached, the entry stays even after
the underlying file leaves the user's library.

After #598's known.met TTL prune drops ~14 k orphaned live entries
on a long-lived profile, those hashes' AICH entries in
known2_64.met are dead weight and should follow them out.

Add CKnownFileList::CollectLiveAICHRoots() -- walks
m_knownFileMap and m_duplicateFileList under list_mut, returns the
set of AICH master hashes still referenced by either. Both lists
need to be scanned: Append's demote branch parks a record (with its
hashset) on the duplicate list while the new record takes the live
slot, and an mtime-restore can re-promote the duplicate later.
Dropping a duplicate's hashset would silently lose it on re-promote.

Extend CAICHSyncTask::Entry()'s existing known2_64.met walk: open a
"<name>.new" temp file via CFile::write_safe, and for each entry
read from the source, either copy it through to the temp (if its
root hash is in liveRoots) or skip it. On clean walk completion the
Close() atomic-renames .new over the original; on corruption catch
or IO error the temp is removed without finalising, leaving the
source's existing truncation-recovery path intact. Hashset bytes
are streamed through a 64 KB buffer rather than slurped, so a
single large-file entry can't dominate the working set.

The dedup root-hash cache (s_rootHashCache, #581) mirrored the old
file; invalidate it after a non-zero drop so the next SaveHashSet
rebuilds against the rewritten known2_64.met.

Effective TTL is inherited from known.met: a record evicted there
by PruneDuplicates ages out of liveRoots and its hashset gets
dropped on the next AICH sync. Decoupled lifecycles would require
bumping KNOWN2_MET_VERSION to add a per-entry timestamp (the file
format is positional, not tag-based), which is out of scope here.

Defensive: if knownfiles isn't yet populated (empty liveRoots), the
prune is skipped -- we don't wipe everything on a misconfigured
start.

…oltip (amule-project#581)

Two IP-filter UX fixes from issue amule-project#580:

- LoadFromFile treated a 0-byte ipfilter.dat as a load failure ("unknown
  format encountered"), because the archive/format detector does not
  recognise an empty file. An empty file is a valid "no ranges" list
  (a user who cleared it, or an auto-update that has not populated it
  yet), so return 0 quietly instead.

- Reword the "Paranoid handling of non-matching IPs" tooltip. The old
  "Use with caution" gave no hint whether caution applied to enabling or
  disabling; the new text says what the check does (anti-spoofing) and
  that disabling it is the risk. Catalogs regenerated via update-po.sh.