Write unit tests for some mg_str functions.

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h/t @hendrikw01

Fixes https://github.com/cesanta/mongoose/issues/826
Closes https://github.com/cesanta/mongoose/pull/831

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Also amend api_net.js to use returned length.

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Raised in previous commit by mistake

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As required by the LWIP API

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With different filesystem implementations and storage drivers, like grown-ups have :)
This is not currently used to add any new functionality, we still have
only one FS and driver on ESP8266 and ESP32; CC3200 has two: SPIFFS and SLFS.
This will be used soon to implement filesystems on additional SPI flash chips and SD cards.

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 - Move keep alive management up to the Mongoose library
 - Only outgoing control messgaes should reset keepalive timer
 - Add unit test

https://forum.mongoose-os.com/discussion/1155/mqtt-keep-alive-compliance-issue

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This adds compile-time checks for format strings.

Fix issues exposed by this.

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Merge commits are not explicitly ignored either: they're just naturally
ignored as empty commits.

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Merge commits are not explicitly ignored either: they're just naturally
ignored as empty commits.

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Fixes https://github.com/cesanta/mongoose-os/issues/260

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ESP32: 2.0-r6
ESP8266: 2.0.0-1.5.0-r5

This brings updated mbedTLS with support for on-disk CA chains:
https://github.com/cesanta/mbedtls/compare/esp32_2.0-r5...esp32_2.0-r6
ESP8266 gets bigger rollup (ESP32 got those changes earlier, seems ok):
https://github.com/cesanta/mbedtls/compare/esp8266...esp8266_2.0.0-1.5.0-r5

Saves ~1.5K RAM for now, but will allow adding more roots to ca.pem without wasting RAM.

Refactored docker build for ESP8266 in the same way as was done earlier for ESP32.

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Other minor fixes.
Also decrease idle poll interval to 100 ms, as a workaround for some cases of incorrect next poll delay computation.

Added HW tests for SSL fetches.

Fixes cesanta/mongoose-os#262

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This prevented hexdump from showing data sent over SSL when using LWIP net_if

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All calls to core LWIP APIs should go through the tcpip thread.

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Rationale: this is the optimisation to make JS FFI-ed API
work nicer. We don't have strucuture introspection now,
and do not generate offsets automatially. Thus, using a
stable well-known location of the parsed message body is
better, in case of number of HTTP headers macro changes.

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Past due timers should be brought forward to restore interval.

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

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As well as URLs without prefix

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