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)

Use PNG if image has alpha layer, JPG if not. Fixes issue #209

Programs called by the key handler might replace symlinks instead of their
targets.

According to the official [Icon Theme Spec](http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#icon_lookup) the Icon key in the desktop entry file should not contain the file extension.