MFC r258135: x86: Allow users to change PSL_RF via ptrace(PT_SETREGS...)

  Debuggers may need to change PSL_RF. Note that tf_eflags is already stored
  in the signal context during signal handling and PSL_RF previously could
  be modified via sigreturn, so this change should not provide any new
  ability to userspace.

  For background see the thread at:
  http://lists.freebsd.org/pipermail/freebsd-i386/2007-September/005910.html

  Reviewed by:	jhb, kib

Sponsored by:	DARPA, AFRL