Convert bup to binary executable and run python subcommands directly

Don't execute python subcommands in a child process, run them
directly.  Convert bup (and dev/python, etc.) to be C executables.
See the commit messages, comments, and doc changes (e.g. DESIGN) for
more information.

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