target_ops/C++: Base FreeBSD target
    
    The
    
      $architecture x NetBSD/OpenBSD/FreeBSD
    
    support matrix complicates things a bit.  There's common BSD target
    code, and there's common architecture-specific code shared between the
    different BSDs.  Current, all that is stiched together to form a final
    target, via the i386bsd_target, x86bsd_target, fbsd_nat_add_target
    functions etc.
    
    Introduces a fbsd_nat_target base/prototype target.  To be used in
    following patches.