add a test for apt-key export{,all}

Git-Dch: Ignore
allow to specify fingerprints in 'apt-key del'
add --secret-keyring option for apt-key

For some advanced usecases it might be handy to specify the secret
keyring to be used (e.g. as it is used in the testcases), but specifying
it via a normal option for gnupg might not be available forever:
http://lists.gnupg.org/pipermail/gnupg-users/2013-August/047180.html

Git-Dch: Ignore
use only one --keyring in gpg interactions

We were down to at most two keyrings before, but gnupg upstream thinks
about dropping support for multiple keyrings in the longrun, so with a
single keyring we hope to be future proof – and 'apt-key adv' isn't a
problem anymore as every change to the keys is merged back, so we have
now the same behavior as before, but support an unlimited amount of
trusted.gpg.d keyrings.