add config value to set full path to cp on POSIX systems

to be able to set a Cmnd_Alias to something like `/bin/cp /home/user/.vagrant.d/tmp/hosts.local /etc/hosts` the command needs to be called with this exact same path, and not only by `cp`(sudo security dictates that full path to binaries must be given).

Furthermore `cp` might live in different locations on different systems.

This pull request is my suggestion on how to fix that. I hope it is useful :)

/Lasse