There are a couple of new packet types in the frisbee protocol which are
exchanged via TCP with the master server: GETREQUEST and GETREPLY.  The
client passes to the master server an opaque imageid and a couple of options
and gets back the addr/port to use to actually download the image.  The
implementation of the master server is fragile and is more of a test
framework, Grant is working on a more robust master server.  I am mostly
doing a backend that communicates with the Emulab DB to do its authentication
and making the client changes.

The client now uses the -S option to specify the IP address of the master
server and the -F option to specify an imageid.  If no error is returned,
the image is downloaded using the returned addr/port.  If -Q is used in place
of -F, then the client makes a "status only" call getting back info about
whether the named image is accessible to the client and whether a server is
currently running.

On the server side, the new master server (mserver.c) has an Emulab
configuration "backend" that supports host-based authentication.
The IP address of the caller is mapped to a node_id/pid/gid/eid combo
that is used to determine access.  On a request, the specified imageid is
treated either as a pathname (if it starts with '/') or an image identifier
of the form "<pid>/<imagename>".  If it is a pathname, we check to make
sure that pathname (after running through "realpath") is contained in one
of the directories accessible to that node in its current experiment context;
i.e., /share, /proj/<pid>, /groups/<pid>/<gid>, or /users/<swapper-uid>.
If it is an image identifier, the DB is queried to ensure that access is
allowed to that image; i.e., it must be "global" or in the appropriate
project/group.

The master server forks a frisbeed for each valid request, if one is not
already running.  The multicast address selection is still based on the
emulab_indicies.frisbee_index field, but the address/port/server info is no
longer stored in the frisbee_blobs table (frisbee_pid, load_address,
load_busy are not set).

Note that this is not yet integrated in the os_load path.  Further work is
required to replace frisbeelauncher.