io_watching_thread(): Declare as a detachable thread, similar to
log_copying_thread().

stop_backup_threads(): Wait for both log_copying_thread and
io_watching_thread to clear their flags. Expect log_sys.mutex
to be held by the caller.

xtrabackup_backup_func(): Initialize log_copying_stop before
creating io_watching_thread. This prevents a race condition
where io_watching_thread() could wait on the condition variable
before it had been fully initialized. This race condition would
cause a hang in the GNU libc implementation of pthread_cond_destroy()
at the end of stop_backup_threads().

This race condition was introduced in
commit 38fd7b7d (MDEV-21452).