Introduce helper to restart kwin on crash exit

Right now when kwin exits, the user is taken directly back to the login
screen. The login session exits, so all processes then are killed by the
session.

This patchset introduces a mechanism to safely restart kwin. The socket
(typically wayland-0) remains alive and persistent across restarts. This
means if any process reconnects through it's own mechanism or a crash
restart handler the socket appears to work, and blocks until the new
kwin restarts.

This makes it secure and race free.

If the screen was locked at the time kwin went down, this is also
secure. Kwin now checks the status from logind at the time of launch, so
will immediately restore a locked state before any other rendering.