Add wrappers around boost::filesystem methods that handle the

Add wrappers around boost::filesystem methods that handle the
required mangling of long filenames on Windows.

Also wrap lots of missing places (e.g. calls to asdcplib, libxml++,
libcxml etc.) in dcp::filesystem::fix_long_path().

The idea is to keep paths un-mangled until they we call some
filesystem-related API and mangle them at that point.  Otherwise
we end up serialising mangled names, which seems like it will
not end well.

Should fix DoM #2623.