shared/install: warn about unkown sections in unit files

As in the previous commit, ignoring unkown sections means users may be confused
easily. It is better to warn about misspellt section names.

In this case, we are using a separate item table, so we'd ignore all those
sections anyway, so we could list them with out the minus prefixes and the
effect would be the same. But I think it's clearer to prefix them.