Split src tree python use to config/bin/python and dev/bup-python

Replace cmd/bup-python with config/bin/python, which is just a symlink
to the configured python, and dev/bup-python, which is what "bup
python" used to be.  Adjust all the code to use config/bin/python when
we can (i.e. when we don't need bup modules), and dev/bup-python
otherwise.  Drop "bup python", since we don't need it anymore.

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