pack: Support --content-only
pack: Support --content-only

This should be useful for implementing more sophisticated packing
strategies.

Also clean up the pack size default handling.

leandrosansilva/ostree,gatispaeglis/ostree,sujitfulse/ostree,resin-io/ostree,leandrosansilva/ostree,GNOME/ostree,sujitfulse/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree,resin-io/ostree,GNOME/ostree,gatispaeglis/ostree,resin-io/ostree,resin-io/ostree,sujitfulse/ostree,gatispaeglis/ostree,sujitfulse/ostree,GNOME/ostree,leandrosansilva/ostree,sujitfulse/ostree,resin-io/ostree,GNOME/ostree,leandrosansilva/ostree,gatispaeglis/ostree