[simulation] enhance simulation for OTNS (#4268)

This commit is to support OTNS by:
- raising the max number of simulating nodes to 999 if OTNS=1
- implementing the status-push mechanism

To build:

$ make -f examples/Makefile-simulation OTNS=1

This commit should have no change of OpenThread behaviors on platforms
except simulation.

examples/platforms/simulation/virtual_time/otns.c

@@ -0,0 +1,50 @@
+/*
+ *  Copyright (c) 2019, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "platform-simulation.h"
+
+#if OPENTHREAD_CONFIG_OTNS_ENABLE
+
+#include <openthread/platform/alarm-micro.h>
+
+void otPlatOtnsStatus(const char *aStatus)
+{
+    struct Event event;
+    uint16_t     statusLength = (uint16_t)strlen(aStatus);
+
+    assert(statusLength < sizeof(event.mData));
+
+    memcpy(event.mData, aStatus, statusLength);
+    event.mDataLength = statusLength;
+    event.mDelay      = 0;
+    event.mEvent      = OT_SIM_EVENT_OTNS_STATUS_PUSH;
+
+    otSimSendEvent(&event);
+}
+
+#endif // OPENTHREAD_CONFIG_OTNS_ENABLE