Consistent handling of shell command quoting

[mih]

As found in #655, we need to deal with command arg quoting in a more consistent manner. Sometimes things are not quoted (leading to failure with, e.g., filenames with spaces), sometimes they are manually quotes (putting arg in "{args}"), sometime sh_quote() is used.

We need to find and document the standard way(s) and implement that throughout.

[mih]

Closed via #690