libusb: avoid hanging when read-thread creation fails #842

hid_open_path() can hang indefinitely when the libusb backend cannot create its read thread. Coverity CID 1706123 (CHECKED_RETURN) identifies the discarded pthread_create() result in libusb/hidapi_thread_pthread.h:153.

At master 16a3ce903683efa27e25c1820e37159d120f7563, hidapi_thread_create() returns void. hidapi_initialize_device() calls it and then unconditionally waits at the two-party startup barrier. If creation returns EAGAIN, no read thread exists to reach the barrier.

Confirmed with fault injection into the current pthread wrapper: forcing pthread_create() to return EAGAIN leaves hidapi_thread_barrier_wait() blocked until a 2-second watchdog terminates the reproducer. This failure does not require concurrent API misuse or hardware-specific behavior.

Expected fix: propagate thread-creation failure, skip the startup barrier and join, unwind the claimed interface/input buffer through the existing cleanup path, and return an open failure. Add a deterministic no-hardware regression covering the error path and successful startup.

The hotplug work in #825 already contains related thread-creation handling. This issue tracks extracting the minimal fix independently onto master; custom HIDAPI_THREAD_MODEL_INCLUDE compatibility must be documented consistently with #829.

Related to #825, #829 and #841. The compatibility notice #829 should remain open.

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