fix: lock users in chroot jail

Please reference comments on asylumfunk/nodeftpd#5 and https://github.com/asylumfunk/nodeftpd/commit/6608b2d06951320759e08fac16650e8d85d3ef3f

This is a heftier commit than I've pushed before, so please ask away if anything is unclear.

I've tested this on both *NIX and WIN machines (as it deals with path separators), but please be diligent as this touches every(?) filesystem access method (implicitly or explicitly).