Minor fixes.

git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@16657 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74
git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@16659 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74
Improved reporting tiff:alpha and allowed setting tiff:alpha to unspecified.

git-svn-id: https://www.imagemagick.org/subversion/ImageMagick/trunk@16660 aa41f4f7-0bf4-0310-aa73-e5a19afd5a74