[wayland] Disallow ptrace on kwin_wayland process

In order to increase the security we disable ptrace on kwin_wayland.
This makes it impossible for a another process running as the same
user to attach to kwin_wayland to install a key logger. It doesn't
protect against higher privileged users, but that's no problem: they
can just read the input device file and don't need to attach to KWin
to become a key logger.

This change is highly inspired by a similar change to kscreenlocker.
A difference is that KWin checks whether we are running under a
debugger. In such a case we still want to allow ptrace.