mbuf: revert part of b4e5a1079d852748f03f32aae87ecdda27a538d2

Restore wlan_seqno, since ether_vlantag has been used by WME.  Fill the
implicit padding on x86_64 and add comment for their possible use.

Reported-by: Johannes Hofmann
i386: Unbreak building if the kern config only contains I686_CPU
i386: Make the system boot again

The CPU_468 etc macros also serve as the indices into i386_cpus[]
make sure that they are interleaved only by 1.
kernel/raid: CAM_NEW_TRAN_CODE is default for over 5 years.
kernel: Remove 3 unused kmalloc types: M_80211_DFS, M_ACPICMBAT, M_ATAPCI.