Remove 64k limit on /dev/shm

Is included in memory limit anyway, no need for a seperate limit. This
was inadvertently reintroduced by the user namespacing changes.

[#82838560]
Move mounting of proc and tmpfs back to child

Mounting in the parent breaks pid isolation.

[#82838802]
Ensure root can write to its own home directory

We have to use an ugly workaround here because aufs won't let us just chown
the directory (the underlying read-only layer's directory permissions
still take effect).

[#82838802]
Make nstar work properly with user-namespaced containers

If the container 'vcap' user's uid differs from the host 'vcap' user's
uid it's important that nstar is in the right namespace so that it
creates files with the right ownership.

[#82838802]