Append behaviour for directory is not as expected

[dnpotter]

Append permissions for a file correctly prevents the file from being overwritten while allowing data to be appended to the file.

Append permissions for a directory should only allow new files to be added to the directory (effectively a file appended to the set of files in the directory). Currently, existing files within the directory can be appended too. This does not seem intuitively right.

Also, consider allowing appended files to be automatically named. e.g. 1, 2, 3 or 0x00..01, 0x00..02, etc