msg/async/Event: use eventfd on Linux

msg/async/Event: use eventfd on Linux

eventfd is Linux's first-choice tool for waking up I/O event loops.
It is faster and more robust than pipes - it does not need a list of
pipe buffers, cannot become full, and writing/reading to it is a
simple memory access.

eventfd has been available since Linux 2.6.22, therefore this patch
does not do any configure-time checks and no runtime fallback.  If
we're __linux__, it must be available.

To avoid an ABI breakage, both fields `notify_receive_fd` and
`notify_send_fd` remain, both set to the same eventfd value.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>