realtime: request SCHED_RR using CAP_SYS_NICE

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

sway/meson.build

@@ -7,6 +7,7 @@ sway_sources = files(
 	'ipc-server.c',
 	'lock.c',
 	'main.c',
+	'realtime.c',
 	'server.c',
 	'swaynag.c',
 	'xdg_activation_v1.c',