cmd/bup: adapt for python 3

cmd/bup: adapt for python 3

Make all the changes necessary for cmd/bup to work with both python 2
and 3.  Given the current state, the majority of the changes handle
untangling the python 3 unicode/data conflation.

Convert to b'x' literals where needed (e.g. for path or path-derived
values), use argv_bytes to convert at least the command line values
that must not be interpreted as locale strings, and switch to the
bytes-only compat.environ.

More broadly speaking, aside from the changes we abosolutely have to
make, the general intent is for us to handle locale-specific
conversions carefully and explicitly when appropriate, not
transparently.

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