kernel - Add safety for Intel SYSRET issue

kernel - Add safety for Intel SYSRET issue

* First, insofar as we can tell DragonFly was *NOT* vulnerable to the
  Intel SYSRET issue.  We have a RQF_QUICKRET flag that determines if SYSRET
  can be used.  Any heavy weight process switch, signal delivery, signal
  return, or set_regs() call clears this flag and forces the system call to
  return via IRET.

* However, the ptrace() path is a bit convoluted.  Insofar as I can tell
  it just won't allow %rip to be changed unless the target process is in
  a SSTOPped state, meaning that a heavy weight context switch must occur
  before the new %rip is used which means we should be safe.

  Still, we are adding a safety to ptrace_set_pc() to cannonicalize the
  %rip anyway, to ensure that this bug cannot bite us indirectly in the
  future.