An empty list (m_) has no pointer so cannot be referenced. Instead we
now return a null pointer (which is no problem for UList)

- added STL-compatible resize() method.
  Should this be the primary entry point?

- made [DS]LListBase end iterators private

- uniform use of reinterpret_cast<foo*>(0) instead of
  reinterpret_cast<foo*>(NULL)

- make all static null() members inline since they are really only a cast:
      "*reinterpret_cast<foo*>(0)"

- The capitalization is consistent with most other template classes, but
  more importantly frees up xfer() for use as method name without needing
  special treatment to avoid ambiguities.

  It seems reasonable to have different names for transfer(...) and xfer()
  methods, since the transfer is occuring in different directions.
  The xfer() method can thus replace the recently introduced zero-parameter
  transfer() methods.
  Other name candidates (eg, yield, release, etc.) were deemed too abstract.

- this should provide a slightly more naturally means to using transfer
  constructors, for example
          labelList list2(list1.transfer());
      vs. labelList list2(xferMove(list1));

- returns a plain list where appropriate (eg, DynamicList, SortableList)
  for example
          labelList list2(dynList1.transfer());
      vs. labelList list2(xferMoveTo<labelList>(dynList1));