Copy stable/10@r272459 to releng/10.1 as part ofthe 10.1-RELEASE process.Approved by: re (implicit)Sponsored by: The FreeBSD Foundation
MFC 259942,262274,263035,263054,263211,263744,264179,264324,264468,264631,264648,264650,264651,266572,267558:Flesh out the AT PIC and 8254 PIT emulations and move them into the kernel.
Move the atpit device model from userspace into vmm.ko for betterprecision and lower latency.Approved by: grehan (co-mentor)