Fix leaking of FDs requested from logind

Summary:
The takeDevice method calls dup, but that syscall removes the O_CLOEXEC flag
as a side-effect. This resulted in all child processes of kwin_wayland having
an open file descriptor for those devices.

Test Plan:
Looked at the open FDs of the startplasma script. Before, all
/dev/input/eventX files and /dev/dri/card0 were open. Now, none of those
are part of /proc/PID/fd/

Reviewers: #plasma, graesslin

Reviewed By: #plasma, graesslin

Subscribers: kwin, plasma-devel, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8887