build-sys: create Makefile-man.am automatically

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.