os-posix: Expand setrlimit() syscall compatibility

Darwin uses a subtly different version of the setrlimit() syscall as
described in the COMPATIBILITY section of the macOS man page. The value
of the rlim_cur member has been adjusted accordingly for Darwin-based
systems.

Signed-off-by: Trent Huber <trentmhuber@gmail.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

os-posix.c

@@ -270,7 +270,11 @@ void os_setup_limits(void)
         return;
     }
 
+#ifdef CONFIG_DARWIN
+    nofile.rlim_cur = OPEN_MAX < nofile.rlim_max ? OPEN_MAX : nofile.rlim_max;
+#else
     nofile.rlim_cur = nofile.rlim_max;
+#endif
 
     if (setrlimit(RLIMIT_NOFILE, &nofile) < 0) {
         warn_report("unable to set NOFILE limit: %s", strerror(errno));