Change priority of karoshi-virtualbox-mkdir PAM config
Add PAM wrapper for pam_exec.so script execution safety

pam-wrapper is a tool that simplifies using scripts with pam_exec.so. It can:
* Drop privilages to PAM_USER for execution
* Eliminate boilerplate for PAM_TYPE
* Correctly produce errors on incorrect input
Modify virutalbox-mkdir to use pam-wrapper
Modify pam-wrapper to correctly set $HOME when dropping privilages
Move *-session scripts to PAM, closes #22

xinitrc has been removed from the configuration, pre-session and post-session
now being implemented as pam-config modules using the new pam-wrapper
interface. Note that no modification to pre-session and post-session was
necessary due to the abilities of pam-wrapper