Client can trick daemon into running server code with am_server == 0

[Matt McCutchen]

Dear rsync people (particularly Wayne),

I noticed that an rsync daemon counts on the client sending a --server
option so that am_server gets set to 1.  If the client doesn't supply
this option, am_server remains 0 but the daemon runs start_server
anyway.  This is potentially dangerous and might lead to a security
hole, although I haven't found one yet.  I suggest that the daemon
either set am_server = 1 explicitly or drop the connection with an
error if the client doesn't supply --server.

Matt