fix greeter environment

there are two distinct problems with doing the greeter start on our own
a) variables that applications expect to be set up and reflect the $user
b) variables that may be set outside sddm's scope containing user specific
   data

to solve both issues the environment setup has been revised to first
adjust all general purpose variables that would be expected to be set
at the minimum. this is primarily to prevent them from being set to the
sddm daemon user values (e.g. daemon HOME=/root, greeter HOME=/var/sddm).
additionally it then removes all known variables that contain user
specific data that are either useless to greeter (SUDO_* variables) or may
contain user specific data in any form (KDE/XDG_HOME variables).

if the mentioned XDG_HOME variables are needed by a theme they must be set
by the theme itself.