CL: Mongoose Web Server: Add UDP client example

Resolves https://github.com/cesanta/mongoose/issues/886

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CL: none

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Integrates https://github.com/cesanta/mongoose/pull/884
Closes https://github.com/cesanta/mongoose/issues/883

CL: Mongoose Web Server: Don't use user_data in mqtt server

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CL: Mongoose Web Server: Fix publish-subscribe example

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Make sure topic is properly NUL-terminated.
Ignore SUBSCRIBE requests with no topic expressions.

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Also make CC3200 use newer ARM compiler, generates slightly smaller code.

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Fixed bug in websocket handshake:
now adding Sec-WebSocket-Protocol header in handshake response.

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This adds compile-time checks for format strings.

Fix issues exposed by this.

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Configured via `debug.std{out,err}_topic`

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It's confusing

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And also fix `flash` target for nrf51

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This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

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