AvNav unnecessarily strict about main member name of .KMZ files

[bobhy]

When adding an overlay to a chart, if you try to use a KMZ file whose main entry is not named "doc.kml", AvNav complains. It is being unnecessarily strict: KML spec at Google Developer says name of main element is "doc.kml" by default, but that's not a mandatory default. I was trying to use this list of tide station locations from NOAA and encountered this unfortunate situation.

I have a workaround for avnav_server, will submit a PR next.