Thanks ploth for coming up with this:
	https://github.com/muennich/sxiv/issues/247#issuecomment-321316306

Replaces optional dependency on xsel(1) to xclip(1) as it supports this.

Fixes issue #247

Pull request #278.

Fixes issue #29 and issue #199.

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.

These formats are also supported by imlib2.

Misbehaving command lines in the key-handler that rewrite the given files and
thus replace symbolic links instead of their targets shall call realpath(1).

Fixes issue #205 and reverts commit 92e3b578.

Related to issue #253, which originally asked for a non-blocking key-handler.

second while maintaining backward compatibiitiy with integer
arguments.

- Functions warn() and die() replaced by GNU-like error(3) function
- Register cleanup() with atexit(3)
- Functions called by cleanup() are marked with CLEANUP and are not allowed to
  call exit(3)