uspace-rtapi: Drop never-used APIs

.. rtapi_task_set_period and rtapi_task_stop are never used.
uspace-rtapi: Introduce class RtapiApp and implementation Pth

.. which has virtual methods that might depend on the userspace
realtime system.
uspace-rtapi: use POSIX scheduler priorities
uspace-rtapi: Implement posix realtime

Some code adapted from the "ubc3" branch (harden_rt)

For SCHED_FIFO, using distinct priorities should give rate monotonic
scheduling.  For SCHED_OTHER (running without root privs), locks give the same
no-slow-interrupts-fast guarantee as pth did (a slightly different guarantee
than RMS, because it also prevents fast interrupting slow).
uspace-rtapi: get time in nanoseconds, not microseconds
build: optionally automatically invoke 'sudo make setuid'

.. touch the file src/.auto-setuid to do this
uspace-rtapi: get rid of use of GNU pth

Since we can now run on posix threads without SCHED_FIFO and provide
the necessary rate-monotonic guarantee, there's no reason to retain the pth
implementation.
uspace-rtapi: lock and page in shared memory segments
uspace-rtapi: cpu_dma_latency hardening