unix, windows: don't treat uv_run_mode as a bitmask

The modes are not meant to be combined, and doing so may hide problems
in the future.

PR-URL: https://github.com/libuv/libuv/pull/58
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
unix, windows: fix UV_RUN_ONCE mode if progress was made

If pending I/O callbacks were ran before polling, do a zero timeout
poll.

PR-URL: https://github.com/libuv/libuv/pull/58
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>