examples/unix/ffi_example: Clean up and update the ffi example.

1. Use uctypes.bytearray_at().

Implementation of the "ffi" module predates that of "uctypes", so
initially some convenience functions to access memory were added
to ffi. Later, they landed in uctypes (which follows CPython's
ctype module).

So, replace undocumented experimental functions from ffi to
documented ones from uctypes.

2. Use more suitable type codes for arguments (e.g. "P" (const void*)
instead of "p" (void*).

3. Some better var naming.

4. Clarify some messages printed by the example.

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