Findadv: Handle \lettrine{} in initials.module

Findadv: Handle \lettrine{} in initials.module

The problem here is, that selecting any subset of a \lettrine{}
line always creates an initials header. That makes it impossible
to our search engine to find strings, because the regex does not
contain that info. So we have to discard the leading \lettrine part
completely.
We place now a marker (\endarguments) to determine that removable
part.