DFXML can't be written to file

[jessek]

The Hashdeep DFXML output mode, -d, always writes the output data to stdout. This means the program ignores the -W mode to write the output to a file.

The problem is that in main.cpp, the call to ocb.xml_open(stdout) always gets called with stdout. This call needs to use the file handle opened by the call to ocb.set_outfilename. This may be difficult, as the file handle may not yet be open the -d flag is processed.