[devtools] fix a couple issues with xvfb-run
xvfb-run didn't quite clean up correctly after itself, because:
- dash doesn't run EXIT traps on signals
- if we don't run the command in the background, a TERM handler doesn't run
until the command exits
debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl