Added public interfaces for better application integration.

lthread_renice() - To allow for thread switching control from
  application code.

lthread_is_lthread_context() - When working in different thread
  contexts it can be necessary to have a way to detect if the current
  thread is running an lthread scheduler without knowing details about
  lthread internals.

lthread_is_eventfd() - This is needed on CentOS 6 to detect the
  lthread event file descriptors from application code.