- fix fibre-specific destructors

- match pthread_getspecific behavior for uninitialized keys

See merge request mkarsten/libfibre!3

pthread_getspecific() returns nullptr when the key has not been
initialized with a pthread_setspecific(). Match this behavior in
libfibre, because this is needed in the Rust port.

- improve stats counter for user-blocking I/O calls (add for epoll_wait)

- clerical updates in fibre.h and syscall_macro.h

  . AtomicSync does not directly support timeouts
  . go back to regular blocking synchronization for I/O readiness notification

  . can be replaced by runtime configuration, if necessary

- clerical update in Stats

- remove TESTING_PLACEMENT_STAGING, TESTING_SHARED_READYQUEUE

- EventScope.h: bug fixes
- IOUring.h: bug fixes
- syscall_macro.h: enhancements

- re-arrange testoptions header files

- minor clerical update in uring

- copy uring's return code to errno
- add stats counters to processor

- clerical updates (stats, ci/cd script, uring)

- additional debug output for scheduling

- clerical updates in liburing compatibility code and poller/uring stats
- streamline code in EventScope::syncIO
- minor cleanup of Fred interface

- make threadtest's signal handler more robust for usage with split-stack
- add more scheduler stats counters

- clerical updates

  (if possible), instead of most recent idle worker in ISRS stack

- clerical updates in src/runtime/Stats.h