Out-of-the-box discovery finds no devices now that libicsneo defaults to Servd

[knakazawa-ics]

Environment

• icsscand master (3.2.0) built per README on Ubuntu 22.04.5
• ValueCAN 4-2 over USB

Symptom

A fresh build of the daemon starts, prints "Waiting for connections...", and never
creates any CAN interfaces.

Since libicsneo switched to Servd-by-default, FindAllDevices() inside the daemon goes
through the Servd path. On a typical SocketCAN host that means:

1. Servd is usually not installed, so discovery silently finds nothing, or
2. even with Servd 26.3.2 installed, discovery is currently blocked by
   Devices discovered via Servd are silently dropped: lowercase serial vs case-sensitive SERIAL_START match libicsneo#89 (lowercase serial vs case-sensitive match), and
3. --filter compares its uppercased argument (uppercased at option parse time in
   src/main.cpp) against Servd's lowercase serials, so filtered discovery would also
   fail once Devices discovered via Servd are silently dropped: lowercase serial vs case-sensitive SERIAL_START match libicsneo#89 is fixed.

Setting LIBICSNEO_USE_SERVD=0 before launching the daemon restores the previous
behavior, and everything works (verified with FIRE 3 and ValueCAN 4-2, including the
listen-only support proposed in #20).

Question / proposal

What is the intended deployment for icsscand going forward?

• If Servd is the intended path: the README should document the Servd dependency, and
  Devices discovered via Servd are silently dropped: lowercase serial vs case-sensitive SERIAL_START match libicsneo#89 plus the --filter case-sensitivity need fixing.
• If the direct driver path is intended: the daemon could set LIBICSNEO_USE_SERVD=0
  itself (or expose a CLI switch), so a stock build works out of the box.

Happy to submit a PR for either direction once you decide.