mv(1) inside the same filesystem was not detected.

Supporting this case made it necessary to always watch the directory. Turns out
the logic and state keeping between arl_setup() and arl_handle() is easier,
when using different watch descriptors for the file and the directory and not
using a oneshot descriptor for the file.

Requiring an absolute canonical path for arl_setup() simplifies dir and base
name splitting. No need for dirname(3) and basename(3) anymore.

Loop reading from inotify fd in arl_handle(); requires non-blocking inotify fd.

No repeated error messages after failed initialization. No error messages on
failed inotify_rm_watch().

Make the header only contain the public interface and nothing from the
implementation. All functions get a handle to their self object, like the img_
and tns_ and win_ functions. All necessary data (file path) is also passed as
an argument, so that no extern redeclarations are needed.

Make arl_setup_dir() private, it's not called outside the module.

Make arl_handle() return true if the file has changed, so that the reloading of
the file can be done by the caller.