unix, windows: return error codes directly

This commit changes the libuv API to return error codes directly rather
than storing them in a loop-global field.

A code snippet like

unix, windows: return error codes directly

This commit changes the libuv API to return error codes directly rather
than storing them in a loop-global field.

A code snippet like this one:

if (uv_foo(loop) < 0) {
uv_err_t err = uv_last_error(loop);
fprintf(stderr, "%s\n", uv_strerror(err));
}

Should be rewritten like this:

int err = uv_foo(loop);
if (err < 0)
fprintf(stderr, "%s\n", uv_strerror(err));

The rationale for this change is that it should make creating bindings
for other languages a lot easier: dealing with struct return values is
painful with most FFIs and often downright buggy.

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windows, unix: remove dead code

windows: rename UV_HANDLE_CLOSING to UV__HANDLE_CLOSING

windows: make active and closing handle state independent

unix, windows: remove handle init counters

Remove the handle init counters, no one uses them.

unix, windows: share uv__handle_init()

windows: inline a couple of handle functions

windows: inline a couple of inline request functions

windows: stop poll watcher when watched socket is closed locally

windows: fix hang when closing an active poll handle

windows: fix bugs in uv__fast_poll_cancel_poll_req

unix, windows: add uv_walk()

Lets the libuv user iterate over the open handles. Mostly intended as a
debugging tool or a post-hoc cleanup mechanism.

unix, windows: rework reference counting scheme

This commit changes how the event loop determines if it needs to stay alive.

Previously, an internal counter was increased whenever a

unix, windows: rework reference counting scheme

This commit changes how the event loop determines if it needs to stay alive.

Previously, an internal counter was increased whenever a handle got created
and decreased again when the handle was closed.

While conceptually simple, it turned out hard to work with: you often want
to keep the event loop alive only if the handle is actually doing something.
Stopped or inactive handles were a frequent source of hanging event loops.

That's why this commit changes the reference counting scheme to a model where
a handle only references the event loop when it's active. 'Active' means
different things for different handle types, e.g.:

* timers: ticking
* sockets: reading, writing or listening
* processes: always active (for now, subject to change)
* idle, check, prepare: only active when started

This commit also changes how the uv_ref() and uv_unref() functions work: they
now operate on the level of individual handles, not the whole event loop.

The Windows implementation was done by Bert Belder.

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Rename uv_platform_socket_t to uv_os_sock_t

Windows: implement uv_poll