core: support MountAPIVFS and RootDirectory in user manager

core: support MountAPIVFS and RootDirectory in user manager

The only piece missing was to somehow make /proc appear in the
new user+mount namespace. It is not possible to mount a new
/proc instance, not even with hidepid=invisible,subset=pid, in
a user namespace unless a PID namespace is created too (and also
at the same time as the other namespaces, it is not possible to
mount a new /proc in a child process that creates a PID namespace
forked from a parent that created a user+mount namespace, it has
to happen at the same time).

Use the host's /proc with a bind-mount as a fallback for this
case. User session services would already run with it, so
nothing is lost.