support gpg 2.1.x in apt-key

support gpg 2.1.x in apt-key

The output of gpg slightly changes in 2.1 which breaks the testcase, but
the real problem is that this branch introduces a new default keyring
format (which is called keybox) and mixing it with simple keyrings (the
previous default format) has various problems like failing in the keybox
to keyring import (#790665) or [older] gpgv versions not being able to
deal with keyboxes (and newer versions as well currently:
https://bugs.gnupg.org/gnupg/issue2025).

We fix this by being a bit more careful in who creates keyrings (aka: we
do it or we take a simple keyring as base) to ensure we always have a
keyring instead of a keybox. This way we can ensure that any version
combination of gpv/gpgv2 and gnupg/gnupg2 without doing explicit version
checks and use the same code for all of them.

Closes: 781042