Enhance set-attr command for access flag

[fadden]

The current set-attr implementation is somewhat primitive:

access may be a two-digit hexadecimal value with the ProDOS access flags, the word "locked", or the word "unlocked".

While this is functional, it's a little awkward to use for anything but the simple lock/unlock case, and it doesn't provide a way to modify some flags without changing others.

It would be useful, especially when operating on a large group of files, to be able to pass arguments like:

• rwnd: set the permission to allow read/write/rename/delete (i.e. unlocked), and clear other bits
• +wnd: add write/rename/delete permission (unlock) without modfying other bits
• -wndb: remove write/rename/delete permission (lock) and clear backup without modifying other bits

Disabling the "read" flag is pretty rare, but the "invisible" flag is set occasionally, and the "backup" bits might be meaningful.