wayland: Make setting realtime scheduler more fine grained

With this only the main and the libinput threads will use realtime
scheduling, so it will be harder to leak realtime scheduling to somebody
else.

The only caveat is that kwin would need to keep CAP_SYS_NICE around,
however on the other hand, it's needed to ensure that kwin_wayland is
able to get high priority EGL contexts with some drivers, e.g. intel.