PUBLISHED_FROM=93fe1cc2c5e2513509511a81e5919cbd37056b10

Frees up a bit of ram on ESP

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`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

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For both OpenSSL and mbedTLS

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PUBLISHED_FROM=7864ffa26cb553f5d4a93248f67c288327ec23b0

Optional specification of SSL/TLS cipher suites.

PUBLISHED_FROM=e8968c6c7a92f10514d3ee575c2fb9be862e6cf8

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

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Currently only OpenSSL API impl, but the plan is to add more.

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PUBLISHED_FROM=3caea25dec11260561349fa096547bd62ecdb509

PUBLISHED_FROM=6455082726781aab819a7f8645e81adaa7fc7669

This event is sent at the end of the request

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PUBLISHED_FROM=017e707ea2bd7b1d1972fbb92e5d98c74e17e9d0

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PUBLISHED_FROM=73ef8ae768585d3c204e6e022848f49e8f4a8de2

Disable sync resolver by default.

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Possible values:
  MG_NET_IF_SOCKET (1) - traditional BSD socket API
  MG_NET_IF_SIMPLELINK (2) - TI's SimpleLink socket interface
  MG_NET_IF_LWIP_LOW_LEVEL - a net_if implementation that uses LWIP's
    low-level API, when LWIP_SOCKET is not available.

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PUBLISHED_FROM=9fd0cebfa8df53b5d9574a8d013305ddda5e7a18

"#if FOO" still works with simple -DFOO, but gives more flexibility.
Specifically, if user expressed no preference (FOO is not defined),
we can apply reasonable defaults (this is the legitimate use of ifdef).

In short, from now on, please use

 #if MG_ENABLE_FOO

instead of

 #ifdef MG_ENABLE_FOO

Since we are all used to #ifdef, this change also adds a precommit check
to police this. Specifically, in *.h and *.c files that are Copyright Cesanta,
"ifdef" and "if defined()" are not allowed to be used with macros that contain
ENABLE or DISABLE, unless the like also contains "ifdef-ok".

Hence, if you are sure you want to use ifdef, use this:

 #ifdef MG_ENABLE_FOO /* ifdef-ok */

PUBLISHED_FROM=9be829448f53cff575d6cae8b9945fb12531c15a

This is the variant of MG_EV_SSI_CALL with context of the SSI call being
processed in addition to the tag argument. Specifically, call handler
now gets access to the HTTP request and the name of the file being processed.

MG_EV_SSI_CALL is preserved for backward compatibility.

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Main TODOs:
1. the spec requires the impl to resend the reply once and
   to send gratuitous updates

2. advertise our clubby services as DNS-SD sub-services.

---

Tested with:

1. https://play.google.com/store/apps/details?id=com.grokkt.android.bonjour&hl=en
2. https://play.google.com/store/apps/details?id=com.druk.bonjour.browser&hl=en
3. osx `dns-sd`

Works well enough; it doesn't have some glitches due the fact that we
don't retransmit yet and we don't send gratuitous updates.

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