kiss-chroot: avoid clobbering resolv.conf and continue cleanup even if umount fails

[TAAPArthur]

This PR contains the following fixes for kiss-chroot kiss-chroot: avoid clobbering resolv.conf

Instead of forcibly overriding the target's resolv.conf, just bind mount
over it. resolv.conf doesn't have to be dynamically generated so the
previous behavior is dangerous.

And

kiss-chroot: continue cleanup even if umount fails

Not every specified directory may have been mounted and in such a case
umount will fail to unmount the dir. Previously this would lead to the
cleanup code existing prematurely and leaving some paths still mounted.
Now these errors are silently ignored.

[dylanaraps]

Made these changes, thanks.