fix lzma-support detection via xz binary

Clear() only clears a config option, not removing it and an empty
setting still exists. Hence we set the option instead to the xz path
so that the later existance check can find a binary for the test
do not ++ on erased package pointers in autoremove

Symptom: In an Ubuntu precise chroot (like on travis-ci)
test-bug-613420-new-garbage-dependency segfaults in a std::set
operator++ on an iterator we have erased previously
(but not if run under gdb of course)
test: use a multiarch capable dpkg rather than workaround

The tests require nowadays a (somewhat) multiarch-capable dpkg, so
replace the workaround as marked in the FIXME with a proper install as
the workaround isn't working always correctly, letting the test fail.

Git-Dch: Ignore
tests: install --no-install-recommends and stunnel4 for travis

stunnel4 is required for https tests

Git-Dch: Ignore