merge fragment keyrings in apt-key to avoid hitting gpg limits

merge fragment keyrings in apt-key to avoid hitting gpg limits

gnupg has a hardlimit of 40 (at the moment) keyrings per invocation,
which can be exceeded with (many) repositories. That is rather
misfortune as the longrun goal was to drop gnupg dependency at some
point in the future, but this can now be considered missed and dropped.

It also means that 'apt-key adv' commands might not have the behaviour
one would expect it to have as it mainly operates on a big temporary
keyring, so commands modifying keys will break. Doing this was never a
good idea anyway through, so lets just hope nothing break too badly.

Closes: 733028