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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It was observed that sl_Send can return SL_EAGAIN, despite the fact that
we only call write_to_socket if it was reported as writeable.

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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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