Pull code for doing a bind mount into separate method

Pull code for doing a bind mount into separate method

The bind mount setup is about to get more complicated.
To avoid having to deal with several copies, pull it
out into a separate lxcContainerMountFSBind method.

Also pull out the iteration over container filesystems,
so that it will be easier to drop in support for non-bind
mount filesystems

* src/lxc/lxc_container.c: Pull bind mount code out into
  lxcContainerMountFSBind