Disable greeters from loading KDE's debug hander

Some themes may use KDE components which will automatically load KDE's
crash handler.

If the greeter were to then somehow crash, that would leave a crash
handler allowing other actions, albeit as the locked down SDDM user.

Only SDDM users using the breeze theme from plasma-workspace are
affected. Safest and simplest fix is to handle this inside SDDM
disabling kcrash via an environment variable for all future themes that
may use these libraries.

CVE-2015-0856