[2829] Improve script of npc_kelerun_bloodmourn

Fix scriptdev2/scriptdev2#48 - Thanks to tarwyn for pointing
Improve Nazan and Vazruden script (first part)
[2830] Improve Nazan and Vazruden script (second part)

Fix scriptdev2/scriptdev2#47 - Thanks to tarwyn and xfurry for pointing and supporting information

Change:
* Add support for nazan to dismount if his health drops below 20%
* Fix Nazan not attacking while he is supposed to fly (previous commit)
* Fix some possible FAIL glitches (previous commit)