LITMUS^RT: take out cpu_online() check for partition parameter

In some plugins, it can be useful to reuse the CPU parameter to denote
virtual CPUs (e.g., containers or reservations). By not requiring a
CPU to be online, we do not impose restrictions on the interpretation
of the cpu field.
LITMUS^RT plugins: add synchronous release callback

Some plugins (e.g., table-driven plugins) may have to do custom setup
when a synchronous task set release occurs. Add an optionall callback
to the plugin interface to allow reacting to synchronous task set
releases.