Refactor mounting of special filesystems

Refactor mounting of special filesystems

Even in non-virtual root filesystem mode we should be mounting
more than just a new /proc. Refactor lxcContainerMountBasicFS
so that it does everything except for /dev and /dev/pts moving
that into lxcContainerMountDevFS. Pass in a source prefix
to lxcContainerMountBasicFS() so it can be used in both shared
root and private root modes.

* src/lxc/lxc_container.c: Unify mounting code for special
  filesystems