More robust escaping of $in, $out paths

In summary: don’t escape if the path doesn’t contain problematic characters, otherwise:
- Shell: enclose string in single quotes, escape embedded single quotes with the magic quote-backslash-quote sequence
- Win32: Escape double quotes by doubling the number of consecutive backslashes that precede them (if any) and adding one more. Finally, double the number of trailing backslashes, and enclose the whole thing in double quotes.
Merge pull request #690 from nickhutchinson/feature/escape-paths-properly

More robust escaping of $in, $out paths