mount-copybind: add MOUNT_COPYBIND_AVOID_OVERLAYFS env var to skip OverlayFS

mount-copybind: add MOUNT_COPYBIND_AVOID_OVERLAYFS env var to skip OverlayFS

In some cases we don't want to even attempt to set up OverlayFS, for
example because SELinux in enforcing mode would kill the process
attempting to use the mount. See:

https://lore.kernel.org/all/CA+FmFJBDwt52Z-dVGfuUcnRMiMtGPhK4cCQJ=J_fg0r3x-b6ng@mail.gmail.com/T/#mef98aa406324096d1889d3d467251f30456f403c

If MOUNT_COPYBIND_AVOID_OVERLAYFS=1 is set, skip directly to copy and
bind mount.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>