Try to gain SCHED_RR (round-robin) realtime scheduling privileges before
starting the server. This requires CAP_SYS_NICE on Linux systems.
We additionally register a pthread_atfork callback which resets the
scheduling class back to SCHED_OTHER (the Linux system default).
Due to CAP_SYS_NICE, setting RLIMIT_RTPRIO has no effect on the process
as documented within man 7 sched (from Linux):
Privileged (CAP_SYS_NICE) threads ignore the RLIMIT_RTPRIO limit;
as with older kernels, they can make arbitrary changes to
scheduling policy and priority. See getrlimit(2) for further
information on RLIMIT_RTPRIO
Note that this requires the sway distribution packagers to set the
CAP_SYS_NICE capability on the sway binary.
Supersedes #6992
