improve verbose message for package_list_command

Instead of only list arg[0] list while command, eg:
old: Reading package list from /usr/bin/env
new: Reading package list from '/usr/bin/env DEBIAN_FRONTEND=noninteractive LC_ALL=C PATH=/bin:/sbin/:/usr/bin:/usr/sbin /usr/bin/dpkg -l
see discussion in: https://groups.google.com/forum/#!topic/help-cfengine/nH6bR-n2x_c