x86_64 - support for AVX instructions

 * CPU will be checked for XSAVE and AVX support on boot. If both are
   found, they will be enabled.

 * If enabled, the kernel will use the XSAVE and XRSTOR instructions to
   save and restore FPU, SSE and AVX registers.

Originally-Submitted-by: Adam Sakareassen (with modifications)
if: Remove IFF_POLLING from kernel space.

User space macro is kept for compatibility.  Adjust comment a little bit.
netgraph7/fec: Don't use IF_ macros on ifnet.if_snd
ppp: Don't use IF_HANDOFF, use IF_ENQUEUE instead.

This nukes the last reference of the IF_HANDOFF
if: Remove deprecated if_handoff and related macros
x86_64/options: Indentation
npx: Always embed saveymm in savefpu and remove dependency on opt_cpu.h

- pcb and mdglobaldata size will not change depending on CPU_DISABLE_AVX
- Modules' makefiles need not to be modified (well, almost all modules)
tun: Don't use IF_DROP on ifnet.if_snd
ef: Don't use IF_ queue macros on ifnet.if_snd
net: Don't use IF_DROP on ifnet.if_snd
iwi: Dont' use IF_ macros on ifnet.if_snd
ral: Don't use IF_ macros on ifnet.if_snd
de: Don't use IF_ macros on ifnet.if_snd
wi: Don't use IF_ macros on ifnet.if_snd
net: Don't use IF_{EN,DE}QUEUE on ifnet.if_snd
if: Update IF_ macros comment
ifq: Update comment in net/ifq_var.h
ifq: Add ifq_{is,set,clr}_started to access/modify altq_started
ifq: Add ifq_get_stage to loacte proper altq_stage data
if: if_start_schedule -> ifq_ifstart_schedule

The argument is changed to ifaltq; this eases further changes to
add multiple tx queues support.