[posix-app] refine OpenThread POSIX-app API (#4073)

This commit makes it easier to OpenThread POSIX-app as a library.

* Remove ot prefix from non-public platform-specific APIs.
* Avoid main() calls non-public platform APIs.
* Parse arguments outside of POSIX-app library.
* Add option to log to stderr.
* Add LOG_OUTPUT flag
* Delete assert() in RadioSpinel::Receive()

tests/scripts/thread-cert/node.py

@@ -78,7 +78,7 @@ class Node:
             cmd = './ot-cli-%s' % mode
 
         if 'RADIO_DEVICE' in os.environ:
-            cmd += ' %s' % os.environ['RADIO_DEVICE']
+            cmd += ' -v %s' % os.environ['RADIO_DEVICE']
             os.environ['NODE_ID'] = str(nodeid)
 
         cmd += ' %d' % nodeid