/*
 * Get the file descriptor of a Fam::Connection object.
 *
 * Note: This method allows you to wait for FAM events using select()
 * instead of polling via Fam::Connection#pending and
 * Fam::Connection#next_event; see the second example below for more
 * information.
 *
 * Aliases:
 *   Fam::Connection#get_descriptor
 *   Fam::Connection#descriptor
 *   Fam::Connection#get_fd
 *
 * Examples:
 *   # simple use
 *   fd = fam.fd
 *
 *   # wrap the FAM connection descriptor in an IO object for use in a
 *   # select() call
 *   io = IO.new fam.fd, 'r'
 *   select [io], , , 10
 *   
 */
static VALUE fam_conn_fd(VALUE self) {