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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Make paths embedded in amalgamated files nice and relative.
This facilitates subsequent clean un- and re-amalgamation.

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On my way, fixed a couple of cases where we had `()` in the header, and
non-empty argument list in the source file.

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Complete rewrite of the C Clubby implementation: it is now modeled after
the Go implementation, with a slight difference that codec and channel
are the same object (something we should probably do in Go as well, as
we only have a single type of channel so far, which is used with all
types of codecs).

This implementation also comes with a new external API,
which is hopefully cleaner and easier to use (see mg_clubby.h).

In this PR I am not adding any new types of channels, but a UART channel
as well as websocket listener channel will be added after this lands.

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A way to serve a single, specific file as opposed to the hairy monster
that is mg_serve_http.

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Remove the random() stubs from the codebase

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Use the same config mechanism for ro_vars as for sys_config, get rid of a bunch of custom code.

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Do not use CA bundle on CC3200, it doesn't work.
Make ssl_ca_cert = "*" mean "no verification" and make sure
SSL is enabled on the connection when it's required, even w/o verification.

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SimpleLink restart invalidates all sockets.
Listeners are restarted, outgoing connections are closed.

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SimpleLink sockets are suffciently different from BSD that all the
ifdefs have become too messy to warrant a separate net_if
implementation. As part of this we also implement proper async connect
support.

Added MG_F_SSL to identify SSL-enabled connections in a generic way,
since SSL state can be different depending on the implementation.

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SL requires cert and key to be separate files in DER format.

Date verification is disabled for now.

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Make restful_server and netcat examples compilation tests:
 - netcat doesn't use HTTP, compile it with MG_DISABLE_HTTP
 - restful_server doesn't use websocket, compile it with
   MG_DISABLE_HTTP_WEBSOCKET

h/t @labourcier, cesanta/mongoose#665

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`v7_mk_undefined()` and `v7_mk_null()` are left unchanged, but marked
deprecated, and `V7_UNDEFINED` and `V7_NULL` are public now.

Xtensa code size with instrumentation: 590400 -> 590432, i.e. 32 bytes.
Xtensa code size without instrumentation: 530848 -> 531760, i.e. 912
bytes.

As objdump reveals, inside v7.c, `v7_mk_number` and others are inlined,
so the extra v7 argument is eliminated; outside of v7.c it's obviously
not inlined.

----

Also, v7_get_int() is added, which currently just casts stored double
value to int

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