duc.c: Create ~/.cache if needed

[licy183]

If ~/.cache doesn't exist, some error will happen. See https://github.com/termux/termux-packages/pull/11350#issuecomment-1193645585.

[l8gravely]

I'd merge this, but you need to handle errors better. If the mkdir() call fails, duc needs to error our more gracefully. I like the idea, but it needs some more work.

And apologies for the delay, now that v1.5.0-rc1 is about to come out, it would be great if you could re-spin this to address these issues.