[s2729] Some small improvements to Zum'Rah

* Rename a function to fit better to needs
* Alter behaviour to work properly in case a grave is looted while the enocunter is in progress
* Do not use currently looted graves by Zum'Rah

(based on commit 4d36785)

Signed-off-by: Xfurry <xfurry@scriptdev2.com>