[z1848] Add SCRIPT_COMMAND_MODIFY_NPC_FLAG to change NPC flags by db-script

Note that the npc flags will be restored when the npc respawns

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

(based on commit 7ec08fc)

(cleaned version of 7ebd504)

(based on commit [s1409] - c11c6eb)