add /usr/share/$project/*-dist.conf to the default config set

This will lookup /usr/share/$project/$project-dist.conf
and /usr/share/$project/$prog-dist.conf
in the correct precedence order by default.
For example from command line utils which don't specify
the --config-file option.  Note if daemon init scripts
explicitly specify a --config-file then they must specify
all required.

For reference the precedence order is that later --config-file
options will override settings from previous ones.
So when no --config-file option is specified, we insert
the "dist" config file as the first to be parsed,
and settings from there are overridden by any from
/etc/$project/$project.conf and /etc/$prog.conf respectively.

For completeness, note settings in configs from --config-dir
take precedence over any of the --config-file settings above.