bup_stat bup_lstat: always treat path as binary

bup_stat bup_lstat: always treat path as binary

Convert the path via "y" in Python 3, and rename the format alias to
cstr_argf to more accurately reflect what it means, i.e. a sequence of
non-null bytes.  If "y" ends up being sufficient it's simpler (and
more efficient?) than a Py_buffer conversion, though it doesn't
support as many argument types.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>