warn if apt-key is used in scripts/its output parsed

warn if apt-key is used in scripts/its output parsed

apt-key needs gnupg for most of its operations, but depending on it
isn't very efficient as apt-key is hardly used by users – and scripts
shouldn't use it to begin with as it is just a silly wrapper. To draw
more attention on the fact that e.g. 'apt-key add' should not be used in
favor of "just" dropping a keyring file into the trusted.gpg.d
directory this commit implements the display of warnings.