Failure to set thread priorities in some devices

[aperezdc]

Reported to happen in a Huawei Mate 40:

WKWebView  D  loadUrl('https://igalia.com')
WPEAndroid D  WKWebView::nativeLoadUrl('...') [tid 10165]
libc       W  pthread_create sched_setscheduler(1073741825, {2}) call failed: Operation not permitted
WPEAndroid D  (process:10165): GLib-ERROR **: 12:15:59.370: 
              file ../glib/gthread-posix.c: line 1340 (g_system_thread_new): 
              error 'Operation not permitted' during 'pthread_create'

The cause seems to be that some vendors do not allow raising thread priorities, reported in mackron/miniaudio#993:

On some Android devices (mostly Huawei et Honor phones), the scheduling policy attribute of a thread cannot be set to SCHEDFIFO (real-time policy) using pthreadattrsetschedpolicy(3) because the main process doesn't have the CAPSYS_NICE which is required (capabilities(7)).

We should check at runtime somewhere during early startup whether raising thread priorities works, and then avoid doing setting priorities higher than the current if needed. Not sure what's the best place to do that... maybe inside WebKit itself?