The Sell All Junk button in the merchant UI never appeared because the
client's C_MerchantFrame.IsSellAllJunkEnabled() reads a sellAllJunkEnabled
CVar whose value is populated from the MirrorVar overlay sent via
SMSG_MIRROR_VARS during authentication. Without this MirrorVar the CVar
defaults to 0 and the button is hidden.

Changes:
- Send sellAllJunkEnabled=1 in SMSG_MIRROR_VARS (AuthHandler)
- Implement CMSG_SELL_ALL_JUNK_ITEMS handler with full item filtering:
  poor quality, sellable, not refundable, not looted, respects per-bag
  ExcludeJunkSell flags and BackpackSellJunkDisabled player setting,
  adds sold items to buyback
- Add SellAllJunkItems packet class (ItemPackets)