For filesystems without high-resolution time support, an usleep() is too short
to alter mtime. Though 1s is quite safe, sleep for 2s in case it gets awaken
right within that second.
The tree.h and queue.h include guards must remain so users don't
accidentally get their system queue.h included as well.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>