man: fix display of keys which appear in two sections in directive index

man: fix display of keys which appear in two sections in directive index

When an index key appeared in multiple sections (e.g.
CPUAffinity= was present in both "SYSTEM MANAGER DIRECTIVES"
and "UNIT DIRECTIVES"), when lxml was used, the key would
be not be displayed in all but one of those sections, and
only an empty <term/> element would be present. This
happens because lxml allows only one parent for each node,
and when the same formatted element was used in multiple places,
it was actually moved between them. Fix this by making a copy
of the element. The bug was present since lxml support was
introduced.

Also fix some indentation issues.