rtapi/posix: try to set SCHED_FIFO, and explain what to do if unable

Posix threads do not guarantee FIFO scheduling (i.e. non-preemption of
higher priority threads) unless the SCHED_FIFO scheduling policy is set.
However, setting SCHED_FIFO requires elevated privileges (SYS_CAP_NICE).

The required capability can be set on the rtapi_app_posix binary by
executing 'sudo setcap cap_sys_nice=pe ../libexec/rtapi_app_posix' .

This requires the setcap program, which can be installed by
'sudo apt-get install libcap2-bin' .

Change this from 'dont try' to 'try, and explain what to do on failure'.

While this is not needed by any of the simulator configs (one thread only),
this change makes tests/threads.0 pass (provided the setcap step is done).