* Linux: fix return value according to return type definitions for some
  ctypes callbacks, thus avoiding "expected type int, got (void 0)".
* Cleaning.
Cleaning.

...trying to less alter "oncommand" attributes, following AMO review.
addEventListener() won't help here (see
http://forums.mozillazine.org/viewtopic.php?f=19&t=2696969).

Note: There seem to be some cases where we would want to keep "oncommand" or
"command" attributes (setting them to "void(0);"), for ex. for <key> elements
(see http://stackoverflow.com/q/16779316/421846).
Bump to version 0.5.2.
Avoid altering 'oncommand' and 'command' attributes (thx Kris Maglione).

Altering on* attributes is discouraged by AMO review, along with the use of
Function() — even if the code passed to Function() is not dynamic.