gen-profile: Read "Exif.Nikon3.*" before "Exif.NikonIi"

1. "Exif.NikonIi.*" are bytes, not even shorts, so they're smaller than
   other keys.

2. That looks like versionned nodes: "Nikon2" vs. "Nikon3".
gen-profile: Add a get_exif_key() function to subr.sh

It avoids to copy exiv2 command line all over the place.