Change to the SSL version of the event scheduler.

  * db/libdb.py.in, xmlrpc/emulabserver.py.in: Only add the testbed
    library path to sys.path if it is not already there.

  * event/sched/GNUmakefile.in: Make the SSL version of the scheduler
    the default instead of the SSH version and statically link the
    executable.

  * event/sched/event-sched.c: Pass the default SSL port number (3069)
    to RPC_init.

  * event/sched/rpc.cc: Bring the SSL code up to date: read the cert
    from the user's home directory, make the connection persistent,
    and use TBROOT as the request path, so the development version of
    the XML-RPC library is used when appropriate.

  * xmlrpc/sslxmlrpc_server.py.in: Updated to let the user select from
    a set of allowed library paths where the 'emulabserver' module
    should be imported from.  Import the 'emulabserver' module after the
    fork so we always get the latest version of the module.  Twiddled
    the necessary bits to turn on persistent connection support.