Added public interfaces for better application integration.

[apohl79]

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.