preliminary ev_async implementation

[Marc Lehmann]

the libev cvs contains a preliminary ev_async implementation, it would be
nice if interested parties could give it a look and try.

it is documented here:

http://pod.tst.eu/http://cvs.schmorp.de/libev/ev.pod#code_ev_async_code_how_to_wake_up_an

basically, it works like an inter-loop signal, that is, no queueing, no data.

we thought long about possibly implementing some lock-free
single-reader/single-writer queue, but I couldn't come up with any good
one, and providing single-reader/multiple-writers is out of the question
it seems.

the manpage provides (untested :) examples on how to do queueing in
signal- and thread-contexts.

feedback (and api changes) are still posssible.

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