utils: refactor g_format_size_full
utils: refactor g_format_size_full

Refactor g_format_size_full to avoid duplicate code and make it easier to
add more units.

https://bugzilla.gnome.org/show_bug.cgi?id=789170

endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib