Implement the handle-based create, queue, priority, cancellation, and
teardown APIs for CONFIG_LIBC_USRWORK. Custom queues use configurable
pthread worker pools while the predefined USRWORK queue remains available.
Match scheduler-backend delay, replacement, cancellation, and lifecycle
semantics. Restrict the libc backend to task context because it uses
blocking synchronization.
Tested on an STM32H7 PX4 FMUv6C with ostest wqueue in Protected user space.
Assisted-by: Codex:GPT-5
Signed-off-by: DuoYuWang <thirteenking.wang@gmail.com>
