shared: remove unused functions

- mkdir_p_prefix: It has never been used
- mkdir_parents_prefix_label: Unused since 1434ae6fd49f8377b0ddbd4c675736e0d3226ea6
man: use <example> instead of multiple <refsect1> for examples
man: move one more nspawn example into a proper <example> section
man: add missing commas
man: fix project reference for archlinux