select: Add defines for EPOLLONESHOT & EPOLLET.
uasyncio: Add checks that IOWrite() syscall return us socket we expect.

One check is commented by default to not hurt performance.
uasyncio: Use EPOLLONESHOT flag for add_reader/writer().

When we issue IORead/IOWrite syscall, we want get back notification just for
that call. But if we add fd to epoll, it will trigger continuously when the
condition is met. For example, a socket with empty write buffer will always
signal EPOLLOUT, regardless if we want to write to it now or not. This will
lead to situation when our coro will be woken up on such socket on *any*
syscall, and this syscall will get completely different socket as result (
or if syscall doesn't return socket - completely different result value).

So, to get semantics right, we need to make sure that for each IORead/IOWrite,
we get notified only once, and further events on socket are ignored until
we ask for them again. This is exactly what EPOLLONESHOT flag does.

The other alternative is to remove fd from epoll after each IORead/IOWrite,
but apparently EPOLLONESHOT is more performant way.

Yet another alternarnative would be to use edge-triggered mode of epoll,
but it has own peculiarities, like, after each event, client must make sure
that it is handled completely and reset, otherwise it may not trigger again,
even if there's unprocessed data. For example, if EPOLLIN|EPOLLET is used,
client must make sure that it reads all data available, until read() returns
EAGAIN. If it reads say just 10 bytes, then next time event simply won't
trigger (because it's edge event, which triggers on change like "no data" -
"data"; if we didn't read all data, the situation is "data" - "data", there's
no change in condition, and event is not triggered). Surely, that's not what
we want (at least not without restructuring how StreamReader works).

So, EPOLLONESHOT is the most obvious, and easiest to reason way to get needed
semantics.