Call a plugin init function by name

Use this macro to arrange init function ordering between friend
plugins.  Fails in the usual manner if the plugin doesn't exist, or if
the init function symbol is AWOL.

clib_error_t *
thisplug_init (vlib_main_t *vm)
{
  clib_error_t *error = 0;

  if ((error = vlib_plugin_init_function ("otherplug.so", otherplug_init)))
     return error;

  <etc>
  return error;
}
VLIB_INIT_FUNCTION(thisplug_init);

Change-Id: Ideecaf46bc0b1546e85096e54be8ddef87946565
Signed-off-by: Dave Barach <dave@barachs.net>