fix: use pthread_self() in thread callbacks instead of parent-stored handle

[aki1770-del]

Fixes #811.

Bug

Two thread callbacks in `src/lib/dlt_user.c` call `pthread_setname_np` with a handle stored by the parent thread:

• `dlt_user_trace_network_segmented_thread` (line 3952): uses `dlt_user.dlt_segmented_nwt_handle`
• `dlt_user_housekeeperthread_function` (line 4838): uses `dlt_housekeeperthread_handle`

Both handles are written by `pthread_create` in the parent. Because the callback begins executing before `pthread_create` returns, the handle is zero or uninitialized at the point `pthread_setname_np` is called. On musl C (standard in many embedded Linux / Yocto configurations), this causes a coredump.

The callbacks are already running in the target thread — `pthread_self()` is the correct and sufficient call.

Fix

```diff

• if (pthread_setname_np(dlt_user.dlt_segmented_nwt_handle, "dlt_segmented"))

• if (pthread_setname_np(pthread_self(), "dlt_segmented"))
  ```

```diff

• if (pthread_setname_np(dlt_housekeeperthread_handle, "dlt_housekeeper"))

• if (pthread_setname_np(pthread_self(), "dlt_housekeeper"))
  ```

Files: `src/lib/dlt_user.c` — +2/−2

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AI-assisted — authored with Claude, reviewed by Komada.