add version comparison commands to Debian
cfengine_stdlib.cf: add package_version_equal_command and package_version_less_command to apt_get and apt_get_release bodies as well
cfengine_stdlib.cf: factor out package_version_equal_command and package_version_less_command to debian_knowledge
Merge pull request #845 from tzz/tzz/bheilman/debian_version_comparison

Add version comparison commands to Debian for all stdlib variations.