More extensive file and path existence checks

[Eyal Rozenberg]

After parsing our arguments, we should check all paths we've been given, ensuring that:

• All directories exist
• All directories with input files (including the kernel source) are executable
• All directories with output files are writable

This is obviously not necessary for the utility to work properly, but will allow for better error messages.

We may also want to check for directory/filename emptiness, just to be on the safe side.