Properly handle propagation options for root and mounts. Now usage of
mount options shared, rshared, and noexec cause error to start. shared/
rshared breaks sandbox=>host isolation. slave however can be supported
because changes propagate from host to sandbox.
Root FS setup moved inside the gofer. Apart from simplifying the code,
it keeps all mounts inside the namespace. And they are torn down when
the namespace is destroyed (DestroyFS is no longer needed).
PiperOrigin-RevId: 239037661
Change-Id: I8b5ee4d50da33c042ea34fa68e56514ebe20e6e0
Fabricio Voznika