pfsync: Dispatch in_{add,del}multi to netisr0

Netisr0 is where multicast is handled.
inet: Dispatch in_ifdetach to netisr0 to run

Currently:
- It only goes through two inpcb lists.  Accessing to these two lists
  is lockless MPSAFE only if the accessing is in netisr0.
- It only modifies inpcbs' multicast option, which should be modified
  in netisr0.
inet: in_{add,del}multi should only be called in netisr0

Add KASSERT to make sure about it.

While I'm here, add blank line before in_addmulti()
polling.4: Add ix(4)
x86_64/ioapic: Enable GSI target CPU auto-balance by default

The original bug, which prevented GSI target CPU auto-balance, probably
was fixed by 902419bf6d9fd0f80afc9d07cd4b3e99d20f23ca.
loader: Remove some commented out code to load modules based on PNP IDs.