build-sys: create Makefile-man.am automatically
man rules were repeating the same information in too many places, which was error prone. Those rules can be easily generated from .xml files. For efficiency and because python is not a required dependency, Makefile-man.am is only regenerated when requested with make update-man-list If no metadata in man/*.xml changed, this file should not change. So only when a new man page or a new alias is added, this file should show up in 'git diff'. The change should then be committed. If the support for building from git without python was dropped, we could drop Makefile-man.am from version control. This would also increase the partial build time (since more stuff would be rebuild whenever sources in man/*.xml would be modified), so it would probably wouldn't be worth it.