[dunecontrol] Properly quote arguments, in particular to exec and bexec.

This means the you can now pass more complex scripts to exec/bexec that use
quoting themselves, and you don't get things like this anymore:
```
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 ---
```

Closes: #51