remove leftover debug output from multikey softlink

Git-Dch: Ignore
rename postrm to apt.postrm

It helps in identifying its affiliation.
Also removes the old postinst which was hidden by apt.postinst for a
long time now and would just install a sources.list in edgecases which
is probably not a good idea (e.g. on my system /etc/apt/sources.list
does not exist). It is better done by the installer of the distro.

Git-Dch: Ignore
remove empty keyrings in trusted.gpg.d on upgrade

Adding and deleting many repositories could cause (empty) keyring files
to pill up in older apt-key versions, which in the end might cause gnupg
to run into its internal limit of at most 40 keyrings
add a (hidden) --quiet option for apt-key
all errors should be printed to stderr

Git-Dch: Ignore
only create new trusted.gpg if directory is writeable
support (multiple) arguments properly in apt-key
set a primary-keyring only if we have access to it
refactor key removal code to reuse it in next step

Git-Dch: Ignore
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
use apt-key adv (+ gnupg) instead of gpgv for verify

apt-key does the keyring merge as we need it, so we just call it instead
of reimplementing it to do the merging before gpgv. This means we don't
use gpgv anymore (we never depended on it explicitly - bad style), but
it also means that the message in apt-cdrom add is a bit less friendly
as it says loudly "untrusted key", but for a one-time command its okay.
correct the error messages to refer to apt-key instead of gpgv

Git-Dch: Ignore
delay gnupg setup in apt-key until it is needed

'apt-key help' and incorrect usage do not need a functioning gnupg
setup, as well as we shouldn't try to setup gnupg before we actually
test if it is available (and print a message if it is not).
support gnupg2 as drop-in replacement for gnupg

If both are available APT will still prefer gpg over gpg2 as it is a bit
more lightweight, but it shouldn't be a problem to use one or the other
(at least at the moment, who knows what will happen in the future).
respect --keyring also in merged keyring commands

Git-Dch: Ignore
use apt-key to wrap gpg calls in testcases

beside testing apt-key a bit it also avoids duplicating gpghome setup
code in apt-key and the test framework

Git-Dch: Ignore