Core/Group: Add sanity checks to CMSG_LOOT_METHOD's handler

If lootMethod or lootThreshold is invalid, portrait menus become unusable to the entire raid.
Core/Group: Fix exploit

The master looter should be in the group.
Core/Group: Any party member may mark targets (this does not apply to raid groups)

Also ensure that the targeted player is not hostile.
Merge pull request #11584 from Dehravor/group

Group related fixes