makefile: install disco in /usr instead of /usr/local
deb pkg: avoid creating the discodb package with the disco package.

discodb has several dependencies which complicates the installation of disco.
Given that it is in a separate repository, we do not need to create its package
with the disco package.
deb pkg: run disco as the disco user.
deb pkg: add info about version 0.5.2
deb pkg: set up ssh before when the package is installed.
Merge pull request #543 from pooya/debian

Debian