between experiments and projects using VMs (jail, vservers, openvz,
etc).

* Add new slot to virt_nodes called "sharing_mode". Only admins can set
  this, via the NS file, to create an experiment consisting of nodes
  that are shared. A text string, in case we want to support different
  kinds of sharing modes.

* Add new slot to reserved called "sharing_mode". This is copied from
  the virt_nodes table as above. Also, virtnodes that are currently
  allocated to a shared node, get this slot set, so that the rest of
  the system knows.

* Add a toklen to the "erole" slot of the reserved table, called
  "sharedhost"; erole is returned to the client when it boots, and
  this is how the client knows it is acting as a shared host. We could
  probably compute this value in tmcd, but I did not want to change
  that now.

* Add new slot to interfaces called "trunk". This is set when an
  interface should be set to trunking by "snmpit -t". We previously
  used the vinterfaces table to determine this, but there are other
  cases when we want an interface to be trunked.

  Aside: there is a problem with this approach I realized later;
  trunked interfaces are essentially shared resources of multiple
  vlans, which does not play well with the vlan optimizations in
  swapmod.

* Add new slot to vinterfaces called "bandwidth"; this is the amount
  of bandwidth that link is defined to use (via the NS file). This is
  used in resource scheduling of link bandwidth.

* Add new slot to interface_state called "remaining_bandwidth". For a
  shared node, we have to reserve both the nodes and the link bandwidth.
  We use this slot (with locking) from the mapper_wrapper to make sure that
  the link has not been consumed already by another assignment running at
  the same time.

* Add new tokens "veth-ne", "veth-en", and "vlans" to the osfeatures slot
  of the os_info table. veth-en (encap) and veth-ne (no-encap) are intended
  to replace the current "veths" feature. "vlans" is set for OSs that
  support vlan devices.

* Fix the primary key on the linkdelays table; it needed to have the
  exptidx in the key so that we can allocated linkdelays for different
  experiments to the same node.