When the source path is not absolute and copy_options::create_symlinks is
specified, deduce the relative path from the target location to the source
file to create a symlink. This allows to copy to a path that is not the current
path.
Also, added absolute overloads taking error_code argyment.
Also, when current_path() is used as a default argument to other operations,
and the operation also accepts error_code, use current_path(ec) to report
errors through the error code rather than throwing an exception.
Also, added a test for copy operation.