Allow to enable realtime priority for the BTS process

I ported the Holger's scheduling patch from sysmobts to osmo-pcu.

This is usefull, if PCU uses direct access to the DSP of sysmobts.
The latency to respond to a PH-READY_TO_SEND.ind may not be higher
than 18ms. Currently we are using nice to increase our priority but
for a heavily loaded cell this is not enough. Add an option to enable
realtime scheduling and use it in the screenrc.

Linux offers two realtime scheduling classes these are SCHED_FIFO
and SCHED_RR. For SCHED_FIFO the process is running as long as possible
(potentially taking all the CPU and never yielding it), for SCHED_RR
the process can still be pre-empted at the end of the timeslice.

Using SCHED_RR appears to be the more safe option as a run-a-way
sysmobts process will not be able to take all the CPU time.

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