Xenomai4 Userspace + other fixes

[hdiethelm]

Besides Xenomai4 Userspace implementation, there are fixes for:

• LXRT was using rt_set_runnable_on_cpus() which is kernel only according to the doc and doesn't work. Changed to rt_task_init_schmod()
• find_rt_cpu_number() was broken. It returned always the last CPU. I found a usefull code snipped in libevl and used it.
• find_rt_cpu_number() used in all uspace implementations
• Cherry-Piked Fix priority check for LXRT #3879 from 2.9 not yet merged to master so I am able to test LXRT

The different uspace modules are mostly copy paste with small changes. I extracted the RtapiApp and Posix class to separate files so it is easier to compare the implementations in a different branch. However, that would make merges from 2.9 to master pretty difficult. I can create a separate MR after that one if desired.