vt(4): Merge several bug fixes and improvements

SVN revisions in this MFC:
  271756 271758 271868 271871 271872 271899

Detailed commit list:

r271756:
  vt(4): Fix out-of-bounds array access in VT_ACTIVATE ioctl handling

  CID:		1229964

r271758
  vt(4): Use strncpy() to copy into a fixed-size buffer

  CID:		1230007

r271868:
  vt(4): Remove vt_buf->vb_dirtymask

  This structure and the associated functions were unused since the
  implementation of vd_bitblt_text_t callbacks.

r271871:
  vt(4): Rewrite history scrolling

  It's now possible to scroll up the 500 hard-coded lines of history, not
  just a fraction of them. For instance, one can reach the top of the boot
  process.

  Sometimes, when scrolling or when changing the screen size (by changing
  the font or loading a KMS driver for instance), one could see the
  history cycling (old content appeared below latest lines). This is
  fixed.

  Now, when the resolution changes are more lines can be shown, the
  displayed area is adjusted so that, if the screen was filled with
  content before, it's filled with content after as well: more history
  is visible, instead of having blank lines below the previously visible
  content.

r271872:
  vt(4): Remove superfluous word in comment

  Submitted by:	brueffer@

r271899:
  Make gcc happy by initialising the variable only set in a couple of
  case statements without a default.

Approved by:	re (marius)
MFC r271726
remove clean option from Makefile

Approved by: re(gjb)
MFC r271762: Switch to text mode in UEFI boot

    The loader previously failed to display on MacBooks and other
    systems where the UEFI firmware remained in graphics mode.

    Submitted by:       Rafael Espindola

Approved by:	re
Spelling, grammar and punctuation fixes.

Approved by:	re (gjb)
Make sure all memory updates are made visible before we let go
of the thread in cpu_switch(). It's otherwise possible that on
another CPU the thread continues from stale context data.

Note that this is prominent on newer CPUs, like the Montecito,
that really take advantage of the weak memory ordering. First
generation Itanium 2 is not that aggressive and does not need
this.

This is a direct commit to stable/10.

Approved by:	re@ (gjb)
MFC 270850,271053,271192,271717:
Save and restore FPU state across suspend and resume on i386.
- Create a separate structure for per-CPU state saved across suspend and
  resume that is a superset of a pcb.
- Store the FPU state for suspend and resume in the new structure
  (for amd64, this moves it out of the PCB)
- On both i386 and amd64, all of the FPU suspend/resume handling is now
  done in C.

Approved by:	re (hrs)
MFC r271727
remove clean option from Makefile

Approved by:	re(gjb)
MFC r271728
For ECORE_DBG_BREAK_IF() ECORE_BUG() ECORE_BUG_ON() check bxe_debug flag before printing error message.

Approved by:	re(gjb)