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 - LWIP UDP sockets are always writable and they are not reported as
   such by select() (bug? featuyre?). TCP sockets are ok.
 - Always deliver MG_EV_POLL, whether we have other fd_events or not.
   Otherwise MG_EV_POLL would not be delivered at all to perma-writable
   UDP sockets (as they are on LWIP).
 - Ignore EAGAIN/EWOULDBLOCK errors while connecting, this happens
   during SSL handshake.

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Until issues with hardware I2C are resolved

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Factor out common part of SPIFFS libc interface support from the CC3200
port and reuse it for ESP32. Minor cleanups.

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This PR does not include `fw/platforms/pic32` since it's too dirty.

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Fix errors so it builds.

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Namely, define it if only `MG_ENABLE_HTTP_STREAMING_MULTIPART` and
`MG_ENABLE_FILESYSTEM` are on

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And use it for `FW_ARCHITECTURE`. This is needed because with
Microchip's xc32, `-DFOO=\"bar\"` results in `FOO` being `bar`, not
`"bar"`, so we have to stringify differently.

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On PIC32MZ, this is required to get `ntohl` and friends.

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Assume handshake request was declined if either MG_F_CLOSE_IMMEDIATELY _or_ MG_F_SEND_AND_CLOSE is set.

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Free keys and cert before conf, not after

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Renegotiation is disabled, so we don't need them.
Reduces per-connection usage when idle by 3K.

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 - Only allow one TCP segment in flight. Each segment is 1.5K and it
   adds up. This may increase latency, but memory is precious on ESP.
   At teh same time, optimize use of tcp_output: do not send a new segment
   on every write, only call tcp_output at the end of connection
   processing during poll.

 - Build LWIP with DNS disabled. We have our won resolver, and LWIP's
   allocates large static buffers (1K).

 - mbedTLS: Deallocate peer's certificate at the end of SSL handshake,
   reduces idle SSL connection footprint by ~1.5K.

 Some tweaks to the heap log viewer (which made all the above possible).

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Also performs trnasparent SPIFFS -> SLFS cert copy.

If the file's extension is .pem, convert it to DER format and put on SLFS.

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Also fixed a memory leak on reconnect: previously, each reconnect was
creating a `struct mg_connection` which was never reclaimed

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Useful for debugging at level 3

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