Restrict slave mountspace so Dalvik apps can mount system-wide volumes

Android 4.2 implements multi-user storage using per-process mount
namespaces.  Everything under "/" (the entire filesystem hierarchy) is
marked as a recursive-slave mountspace for all zygote instances.  This
is done so that user-storage sandbox mounts under /storage/emulated are
hidden from other apps and users.

Unfortunately this means that any Dalvik app (actually, any program
whose clone/fork ancestry includes a Dalvik zygote, which is everything
except services spawned directly from init) cannot mount system-wide
volumes. Thus, apps like CifsManager are effectively broken in Android
4.2, since its cifs mounts are only visible to the CifsManager app
itself.  All other apps see empty mountpoints instead of the mounted
volume.  Furthermore, Linux provides no provision for a process to
"escape" a recursive-slave mountspace in versions prior to Linux 3.8
(setns syscall).

Here, we restrict rootfs-slave calls to /storage (and, due to a possible
kernel bug, /mnt/shell/emulated) so that Dalvik apps can mount
system-wide volumes elsewhere (with appropriate permission, as in
earlier versions of Android), while retaining full multi-user storage
compatibility.  It is made available as a kernel-based workaround for
instances where libdvm can not be modified.

This change requires that a tmpfs volume is mounted as /storage in
init.rc. If this volume is unavailable, then we falls back to the
previous behavior of marking the entire filesystem hierarchy as slave.