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Implement Linux-compatible TFD_TIMER_CANCEL_ON_SET flag for timerfd to allow applications to detect discontinuous changes to CLOCK_REALTIME. Background: According to Linux timerfd_create(2) man page, when a timerfd is created with CLOCK_REALTIME and TFD_TIMER_CANCEL_ON_SET flag is specified, the read() operation should fail with ECANCELED if the real-time clock undergoes a discontinuous change. This allows applications to detect and respond to system time changes. Implementation: 1. Add clock notifier infrastructure (clock_notifier.h/c) to notify interested parties when system time changes 2. Implement TFD_TIMER_CANCEL_ON_SET flag support in timerfd 3. Register timerfd with clock notifier when flag is set 4. Cancel timer and return ECANCELED when clock change is detected 5. Notify clock changes in: - clock_settime() - clock_initialize() - clock_timekeeping_set_wall_time() - rpmsg_rtc time synchronization Changes include: - New files: include/nuttx/clock_notifier.h, sched/clock/clock_notifier.c - Modified: fs/vfs/fs_timerfd.c to handle TFD_TIMER_CANCEL_ON_SET - Modified: clock subsystem to call notifier chain on time changes - Modified: rpmsg_rtc to notify time changes during sync Use case example: Applications using timerfd with absolute time can now detect when system time is adjusted (e.g., NTP sync, manual time change) and take appropriate action such as recalculating timeouts or updating scheduled events. Reference: https://man7.org/linux/man-pages/man2/timerfd_create.2.html Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>