Minor improvement to the node.available() method so you can get the
number of free nodes of a particular type (e.g. pc850).

  * xmlrpc/GNUmakefile.in: Add node_avail symlink.

  * xmlrpc/emulabserver.in: Add some optional parameters to
    node.available() so you can specify the class/type of nodes as
    well as project credentials.

  * xmlrpc/script_wrapper.py.in: Add node_avail class that calls
    node.available() with the given parameters.