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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