[z1991] More db-script command changes

* Allow SCRIPT_COMMAND_KILL_CREDIT to give the kill of an involved npc.
  This is marked by datalong (creature-entry) == 0
* Remove immediate execution of commands. This will prevent infinite loops when db-scripts trigger db-scripts which are executed immediately
* Add check for SCRIPT_COMMAND_PLAY_SOUND
* Add check to catch an invalid command

Signed-off-by: Schmoozerd <schmoozerd@scriptdev2.com>

(based on commit 1bf4280)

Signed-off-by: Nighoo <nighoo@gmail.com>

(cleaned version of 29054c2)

(based on commit [s1579] - 0b2e29c)