DLD upload: Avoid using open_memstream() and non-portable functions

patch contains:
- replacement for open_memstream()
- storage of the temporary zip file in the OS temporary directory
- replacement usage of mktemp() with g_mkstemp()

patch based on work by Miika Turkia.

[Dirk Hohndel: as suggested by Miika I commented out the deletion of the
               DLD file]

Acked-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>