M4 & MK18 Fix

Fix for the original RiverX_96 mod, "M4 & Mk18 with attachments" found here: https://reforger.armaplatform.com/workshop/5965770215E93269-M4%2526MK18withattachments

Updated slots for rail attachments.
Added bottom rail slots to both guns.
Added carryhandle optic slot to M4.
Weapons can be found in US Arsenals/conflict weapon boxes/weapon racks.