Rework thread priority management

[Sebastian Huber]

This patch set reworks the thread priority management.  The main goals are

1. an enhanced implementation of the priority inheritance protocol, and

2. support for the OMIP locking protocol.

Instead of the current and real priority values in combination with the
resource count use thread priority nodes which contribute to the overall thread
priority.

The actual priority of a thread is now an aggregation of priority nodes.
The thread priority aggregation for the home scheduler instance of a
thread consists of at least one priority node, which is normally the
real priority of the thread.  The locking protocols (e.g. priority
ceiling and priority inheritance), rate-monotonic period objects and the
POSIX sporadic server add, change and remove priority nodes.

A thread changes its priority now immediately, e.g. priority changes are
not deferred until the thread releases its last resource.