Fixes:
------

- Fix two minor bugs in clock and POSIX timer handling.

- Fix an ordering problem in sys/termios.h.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-July/258474.html

- Fix doxygen hang due to a deadlock between fclose() and fork().
  Addresses: https://cygwin.com/pipermail/cygwin-apps/2025-May/044318.html
  Addresses: https://cygwin.com/pipermail/cygwin/2025-June/258323.html

- Fix multi-thread safety of system().
  Addresses: https://cygwin.com/pipermail/cygwin/2025-June/258324.html

- Revert fix handling of invalid 4 byte UTF-8 sequence. It was broken.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-July/258513.html

- Instead, fix internal conversion of filenames in case of an invalid
  4 byte UTF-8 sequence.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-June/258358.html

- Make process_fd correctly handle pty and console.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-May/258167.html