Revert "force unmount; lazy was not enough"

This reverts commit feb713ea6c263f8b85e0e47250281198943a35aa.
Revert "umount with MNT_DETACH"

This reverts commit d8bb94eb0befb344db2b39d2622fc2e13afa0f39.
simply umount the pivoted root with MNT_DETACH

no longer recurse through the sub-mount points; this is prone to error if e.g.
a mount point's source is deleted. in this state, some kernels will not allow
it to be umounted.

this apprach is much simpler and seems to be just as safe (none of the mount
points are visible in the child).