graphics/ImageMagick7: Convert -nox11 to flavor
graphics/ImageMagick6: Convert -nox11 to flavor
Handle fallback for version and flavor separately, also add several
sanity checks.
This fixes creating broken *_DEPENDS for ports using it without
arguments when a -nox11 version is requested in DEFAULT_VERSIONS like
e.g. this resulting from "imagemagick=7-nox11":
libMagick++-7-nox11.so:graphics/ImageMagick7-nox11