Excluding lightdm process from list

The display manager LightDM has known bugs in Debian based distros.
Debian: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=675351
Ubuntu: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/993573 and ://bugs.launchpad.net/lightdm/+bug/990661
For this reason, a safe strategy i to exclude it from zombies proccess list.
Merge pull request #4 from maurobaraldi/master

Excluding lightdm process from list