Migrates to using os_log for app logging on iOS 13 and above, macOS
10.11 and above. On older platform, fall back to syslog(), which is what
we used previously.
This re-lands commit 78a8909725bbaeec80870f498d01ea6e56932a3a with a fix.
That commit was reverted in a61dbf2f66b97e85f4d8bf0cfb29a8b3c2640c09.
Migrates to using os_log for app logging on iOS 13 and above, macOS
10.11 and above. On older platform, fall back to syslog(), which is what
we used previously.
