[vm/io] Make sure to write until EAGAIN on POSIX

We are using `epoll()` in edge-triggerred mode, which requires
us to hit `EAGAIN` on `write()` before we are guaranteed
to receive an `EPOLLOUT` event.

`_SocketStreamConsumer` looks at `_NativeSocket.writeAvailable`
after issuing a `write` to decide whether it should continue
writing or pause and wait until event handler sends us `writeEvent`.

However we did not previously set `writeAvailable` to `true` until
the first `writeEvent` arrived, which lead to a hang on some
Linux servers: we would write a small amount of bytes into a pipe
and then wait for `writeEvent`, which would never come, as `epoll()`
is only guaranteed to wake up with `EPOLLOUT` if we hit `EAGAIN`
on `write()`.

This CL also changes `_NativeSocket.nativeWrite` implementation
to write bytes into the file descriptor until it gets EAGAIN.
This reduces the number of roundtrips between Dart and
native.

Unfortunately this CL does not come with a regression test
because it is relying on concrete `epoll()` behaviour and it this
behaviour does not reproduce for me.

TEST=Tested manually on the affected servers.

FIXED=b/286346121

Fixed: 286346121
Change-Id: I37fef9aa12b1da724b035aa9577b414a8057217e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/308101
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>