add testcase for APT::Architectures "arch1,arch2"

Git-Dch: Ignore
enable fvisibility=hidden for our private library

While it is a huge undertaking to enable it for our public libraries as
basically everything we exported so far could be seen as public
interface our private library is new and under our full control, so we
can do whatever we like with it. The benefits are not that big in return
of course, but it reduces the size a bit, so thats great nontheless.

Git-Dch: ignore
mark optional (private) symbols as hidden

This methods should not be used by anyone expect the library itself as
they are helpers for the specific class and therefore perfect candidates
for hidding.

Git-Dch: Ignore
ensure proper teardown in dpkg error cases

We have to properly close our pseudo terminals even in error cases
before we call post-invoke scripts. This is done now by breaking from
the dpkg calling loop instead of copying the handling, which did it in
the wrong order before.

This also ensures that our state file is written in error cases to
record autobit and co as this was forgotten before.

Closes: 738969
update symbols file to include new symbols from 0.9.16