Python IP addresses: Check for and maybe delete stale ipaddr.py

If ipaddr.py (or .pyc) from a previous secnet installation still
exists in /usr/share/secnet or /usr/local/share/secnet, this version
won't work.  Check for this situation in `make install' and bomb out.
Provide a `make install-force' which deletes the spurious files.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>