Cleanup endian support (closes #271)

- Place common code under OSspecific.

By including "endian.H", either one of WM_BIG_ENDIAN or WM_LITTLE_ENDIAN
will be defined.

Provides inline 32-bit and 64-bit byte swap routines that can be
used/re-used elsewhere.

The inplace memory swaps currently used by the VTK output are left for
the moment pending further cleanup of that code.