poll,unix: ensure safety of rapid fd reuse

Consider the following scenario:

uv_poll_init(loop, poll, fd);
uv_poll_start(poll, UV_READABLE, cb);
// the cb gets invoked etc.
uv_poll_stop(poll);

close(fd);
fd = allocate_new_socket(); // allocate_new_socket() is assigned the same fd by "bad luck" from the OS

// some time later:
uv_poll_init(loop, otherpoll, fd);
uv_poll_start(otherpoll, UV_READABLE, cb);

uv_close(poll); // uv__io_stop: Assertion `loop->watchers[w->fd] == w' failed.

According to documentation, "however the fd can be safely closed
immediately after a call to uv_poll_stop() or uv_close()."
Though, in this scenario, we close()'d our file descriptor, and by
bad luck we got the same file descriptor again and register a new
handle for it and start polling.

Previously that would lead to an assertion failure, if we were to
properly free the original handle via uv_close().

This commit fixes that by moving the check whether a only a single
poll handle is active to uv_poll_start() instead of the stopping
routines.

Fixes: https://github.com/libuv/libuv/issues/1172
Fixes: https://github.com/bwoebi/php-uv/issues/81
Fixes: https://github.com/b2wdigital/aiologger/issues/82
Fixes: https://github.com/invenia/LibPQ.jl/issues/140
PR-URL: https://github.com/libuv/libuv/pull/2686
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>

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