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Fixed a bunch of things to make OTA work on ESP32 with flash encryption:
writes to app aprtitions must be 32-byte aligned and mod 32 in size.
When merging filesystems during update, use spiffs_vfs_* functions so
that old fs is properly decrypted.

Refactored cs_dirent stuff: SPIFFS support moved to spiffs_vfs.c,
added dirent.h on ESP8266 and CC3200 which includes cs_dirent.h
Define DIR and dirent only if asked (ESP8266, CC3200, WIN32).

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cesanta/esp-idf is rebased onto espressif/esp-idf#aece3d6fc62fd91d3c5a87e0ab2ddb5ac51d8ff0

A few wifi changes and VFS support for directory-related functions.

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Optional specification of SSL/TLS cipher suites.

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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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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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