Improve Debian bash-completion support

* Autoconf queries pkgconf for bash-completion dir-- resolves
  lintian's `package-install-into-obsolete-dir` diagnostic
* Use improved completion snippet from
  <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=782169

>
* Remove old /etc/bash_completion.d/mosh "conffile" with
  dh-maintscript + dpkg-maintscript-helper

Closes #628.

Signed-off-by: John Hood <cgull@glup.org>