Shkarko NSClient++ 0.26.0

# Nagios NCPA support, a check that finds what is filling a disk, and scripts that run from the fleet tree

0.26.0 adds two experimental pieces: an `NCPAServer` module that lets Nagios' `check_ncpa.py` poll NSClient++, and `check_largest_files`, which ranks the files or folders filling a disk and reads the NTFS Master File Table on Windows. A new `script root` setting lets every script module run its scripts from the fleet tree. The WEB server now uses its `threads` pool, keeps connections alive and no longer falls back to cleartext when its certificate does not load. Missing predefined performance counters are retried until they exist, and a fleet install stops writing local configuration.

## ✨ Highlights

- 📡 **NCPAServer: poll NSClient++ with `check_ncpa.py`.** A new, experimental module serves the Nagios NCPA HTTP API on port 5693, token-authenticated and limited by `allowed hosts`. This release serves the `plugins/` node, which runs any check, alias or script (#1633).
- 💾 **`check_largest_files` finds what is filling a disk.** It lists the `top` largest files (or, with `type=folders`, folders) beneath a path. On Windows it reads the NTFS Master File Table, which takes seconds where walking a large volume takes minutes (#1692).
- 🛰️ **`script root` runs scripts from the fleet tree.** External, Lua, Python and check_mk scripts can all load from `${fleet-folder}/scripts`, a shared folder or one of your own, so a fleet bundle can ship its scripts (#1673, #1684).
- ⏱️ **The WEB server serves requests in parallel and keeps connections alive.** `threads` (default 10) finally sizes a real pool, so one slow REST check no longer holds up the web UI. Connections are reused, and the request body cap is now the `max request size` setting (#1653, #1678).
- 🔒 **No cleartext fallback when the web certificate does not load.** An unreadable or mismatched certificate or key stops the WEB server with a reason. It no longer serves plain HTTP on 8443 or fails every handshake (#1633).
- 📊 **Late performance counters are picked up.** One predefined counter that was not registered yet used to break every predefined counter until a restart. Each one is now retried on its own until it appears (#634, #1656).
- 🛰️ **A fleet install writes no local configuration.** An MSI install with `FLEET_SERVER` only adds the fleet include, so the fleet server's settings are no longer overridden by installer defaults (#1688).

## ⚠️ Upgrade notes

The full list is on the [upgrading page](https://nsclient.org/docs/setup/upgrading/), and the security fixes are on the
[security notices page](https://nsclient.org/docs/security/notices/).

- **WEB server certificate.** If the configured certificate or key does not load, the WEB server no longer starts. Check the log after upgrading and fix the file, or deliberately opt into cleartext.
- **Fleet installs.** Configuration passed to an MSI install alongside `FLEET_SERVER` is now ignored. Move those values to the fleet server, and have it enable the modules it needs. Hosts that are already enrolled keep their `nsclient.ini`.
- **Hand-rolled Windows deployments** that copy the DLLs next to `nscp.exe` must add `boost_json.dll`.
- **`check_process` `pagefile` / `peak_pagefile`.** Thresholds written against the swapped values should switch keywords. Graphs of `pf_use` / `ppf_use` flip at the upgrade.
- **Linux and macOS external scripts** now always start in the folder above the scripts folder. The systemd unit already did this. Agents started by hand or as a container entrypoint may see a different working directory.
- **Log-matching rules.** A missing performance counter now logs `Failed to add counter <name> (<path>): ...` instead of `Exception in Failed to add counter`. A predefined counter read in the first seconds after startup answers `Failed to get value` instead of `0`.

**Full Changelog**: https://github.com/mickem/nscp/compare/0.25.0...0.26.0