Although writing to a proxy is primarily intended for oob callers,
allowing inband tasks to use this channel too comes in handy as the
sequence of messages relayed by such proxy can be preserved regardless
of the execution stage they originate from, as they percolate through
the same buffer.

Inband writers can block until the output has drained, unless
O_NONBLOCK is set for the proxy.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>