Improve app data utility module
Improve app data utility module

Follow Python's recipe for platform identification (startswith).
Add freebsd as an assumed-posix platforms (but it's untested).
Rename the verb in the data shelve procedure to "open"
so it better fits Python's context manager (the with statement).
Add an exception if this module is run on an unsupported platform.

dwhall/HeyMac,dwhall/HeyMac