Fix error handling in copy_file and equivalent

Fix error handling in copy_file and equivalent

	* src/filesystem/ops.cc (do_copy_file): Report an error if source or
	destination is not a regular file (LWG 2712).
	(equivalent): Fix error handling and result when only one file exists.
	* testsuite/experimental/filesystem/operations/copy.cc: Remove files
	created by tests. Test copying directories.
	* testsuite/experimental/filesystem/operations/copy_file.cc: Remove
	files created by tests.
	* testsuite/experimental/filesystem/operations/equivalent.cc: New.
	* testsuite/experimental/filesystem/operations/is_empty.cc: New.
	* testsuite/experimental/filesystem/operations/read_symlink.cc: Remove
	file created by test.
	* testsuite/experimental/filesystem/operations/remove_all.cc: New.
	* testsuite/util/testsuite_fs.h (~scoped_file): Only try to remove
	file if path is non-empty, to support removal by other means.

From-SVN: r241521