Revert "mount-setup: change system mount propagation to shared by default"

This reverts commit b3ac5f8cb98757416d8660023d6564a7c411f0a0.

Conflicts:
	TODO
	src/core/mount-setup.c

We want mounts to be private, complying with the standard kernel behavior.
Shared mounts interfere with pivot_root and makes namespace leak. Making
all mounts shared by default was a massive mistake that breaks userspace.

We lost about 20 hours of combined work time thanks to this "feature".