testing/ejabberd: stop using aports git as version & assume maintainership

ejabberd's version and directory name were wrongly taken from aports git, also libdir has been moved from /usr/lib to /usr/lib/ejabberd, to avoid polluting /usr/lib.

bash-completion subpackage, standard autoconf options, and --enable-group=ejabberd have been added.

Default captcha script has been moved to -doc examples, because it depends on imagemagick (which is not in depends=), and using the default without any modifications increases the chances of the captcha challenge being solvable by bots.

Various problems with vendored Elixir have been solved.

Lastly, assuming maintainership because original maintainer has not been active since 2015.