Add the less-ambiguous freebsd-version command.

Reviewed by:	-stable
MFC after:	3 days
Followup to r273966.  Fix the build with ADAPTIVE_LOCKMGRS kernel option.

Note that the option is currently not used in any in-tree kernel
configs, including LINTs.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Don't check for a NULL curthread.  curthread hasn't been NULL after early
boot since 5.0.
Remove the FP_SOFTFP flag.  It wasn't used but was leftover from the
software x86 math emulator.
MFamd64: Explicitly initialize the mxcsr during npxinit().
MFamd64: Move extern declaration of _ucodesel and _udatasel to
<machine/md_var.h>
Fix a bug which prevented ND6_IFF_IFDISABLED flag from clearing when
the newly-added IPv6 address was /128.

PR:	188032
MFamd64: Add support for extended FPU states on i386.  This includes
support for AVX on i386.
- Similar to amd64, move the FPU save area out of the PCB and instead
  store saved FPU state in a variable-sized buffer after the PCB on the
  stack.
- To support the variable PCB location, alter the locore code to only use
  the bottom-most page of proc0stack for init386().  init386() returns
  the correct stack pointer to locore which adjusts the stack for thread0
  before calling mi_startup().
- Don't bother setting cr3 in thread0's pcb in locore before calling
  init386().  It wasn't used (init386() overwrote it at the end) and
  it doesn't work with the variable-sized FPU save area.
- Remove the new-bus attachment from npx.  This was only ever useful for
  external co-processors using IRQ13, but those have not been supported
  for several years.  npxinit() is now called much earlier during boot
  (init386()) similar to amd64.
- Implement PT_{GET,SET}XSTATE and I386_GET_XFPUSTATE.
- npxsave() is now only called from context switch contexts so it can
  use XSAVEOPT.

Differential Revision:	https://reviews.freebsd.org/D1058
Reviewed by:	kib
Tested on:	FreeBSD/i386 VM under bhyve on Intel i5-2520