Initial commit for the dxfout plug-in.
Added the dxfout target.
Added file handling and writing a report to stderr.
Added dxf_write_block() and needed defines and globals.
Added defines for DXF.
Added dxf_clean_string().
Added the first walk of Elements for a block rereference list.
Added a placeholder for dxf_write_table_block_record().
Added dxf_write_comment().
Added a lot of code.

- structs for StringList and DxfList
- dxf_string_insert() function
- dxf_insert() function
- dxf_write_endsection() function
- dxf_write_header_metric() function
- dxf_write_section() function
- and a lot of other code.