Bug#773775: Fix and improve progress-linux archive key installation

[jnqnfe <jnqnfe@gmail.com>]

Small mistake: Somehow in mulling over the possibility of also adding
gpg & gpg2 to the list of possible tools to use (which I ditched), I
mistakenly came under the impression that the --no-default-keyring
parameter was available on gpgv/gpgv2, but it's not, so it needs
removing from the patch I submitted, if adopted, otherwise it results in
an error.

I also noticed that in fact there's already a patch on the debian-next
branch addressing the progress-linux file/folder/release renaming
aspects, and covers more of them than me. Though mine does fix a
possible important apt-key add failure issue (as described in previous
message), and some other improvements. Perhaps both could be merged
together, or whatever...