dunecontrol: improper quoting of command arguments

This means you are limited to very simple skripts in dunecontrol exec/dunecontrol bexec that don't use quotes themselves.

joe@paranoia:~/Projekte/dune-quote/dune-common$ LC_ALL=C bin/dunecontrol exec 'var="quoted value"; echo "$var"'
--- going to build dune-common  ---
--- calling exec for dune-common ---
bash: value: command not found

--- dune-common done ---
--- done ---