Support actually building a Debian package

We had a debian directory with an init script and a defaults file in it, but not all the stuff required to actually build a Debian package. This commit adds them, and makes a few supporting changes.

* The debian-install makefile target is now just "install" because it doesn't do anything Debian-specific - creating a user/group is moved to the postinst script.

* The install location is now relative to the DESTDIR makefile variable.

* Logs go into /var/log/fanout, which is owned by the fanout user.

* The binary goes into /usr/sbin, not /usr/local/bin, which seems more appropriate

* The "clean" target now checks for the existence of the binary before deleting it, to prevent a bad return code from causing make to error out