1. Make GHC work with paths and filenames containing spaces (hopefully).
This is done by quoting all arguments sent to the shell. To avoid causing
problems, this means that each argument in a list had *better* be a single
argument, hence the next commit to PackageSrc.hs.

2. undosify the path passed as the -h argument to unlit, to make unlit work
on Windows again.

3. Fix getExecDir for Windows (broken by previous changes to the meaning of
-B).