Added fake /usr/bin/0alias, etc for unit-tests

Before, some tests only worked if 0install was already installed.
Add install_local command to make
Suggest installing to /usr/local by default

We no longer need to be in /usr, because the Python code has all gone.

Also, removed unused $PYTHON and $DESTDIR tests. Was causing:

./install.sh: line 34: DESTDIR: unbound variable