Solution: build-depend on asciidoc and xmlto

Solution: create config subdir as a workaround if building the
packages with automake < 1.14

Solution: depend on libunwind-dev | libunwind8-dev | libunwind7-dev

Solution: update NEWS, include/zmq.h and packaging

Solution: parse the OBS prjconf and the user env variable
DEB_BUILD_OPTIONS. If either of those define "drafts" then build with
draft APIs enabled.

Solution: import and adapt Debian's packaging code from
https://packages.debian.org/source/unstable/zeromq3
As noted in packaging/debian/copyright, these files are licensed
under the LGPL2+ and the copyright belongs to the authors listed in
the same file.