allow specifying name when writing SGI-images

(not used yet)
on the way to write-support for imageSGI
announce SGI write support
do not free "lptr" pointer

as this is only a temporary, that points to "base" (which is already freed)
add imgname to longstoimage()
set image-name

defaults to "Gem image"
added "imagename" property
make getLongImage() return early on failure