Basically moving PBD::copy_file implementation to pbd/filesystem.h/cc.
The implementation itself looks like it could be improved to use
much less memory when copying big files by reading and writing in
chunks but I don't think that is an issue at present.
git-svn-id: svn://localhost/ardour2/trunk@2376 d708f5d6-7413-0410-9779-e7cbd77b26cf