[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 (closed)

Planned merge date: 2016-11-04 or later.

I plan to merge this on or after 2016-11-03.

Added 98 commits:

• f80bcf23...d36cdb63 - 97 commits from branch master
• 6455bc0c - Merge branch 'master' into feature/proper-quoting-for-dunecontrol-exec-args

The patch failed CI because it was against an older master that still failed the parametertree test. pulled master into this branch; that will probably fix it.

I'm pushing the planned merge back by one day because of this: will now merge on or after 2016-11-04.

Status changed to merged

Mentioned in commit 6e4a9c39

Without testing it: Thanks for this improvement!