In windows libuv does not return -errno, instead it uses negative
error codes e.g. UV_ENOENT. This commit changes the comments in os_*
functions to reflect this.

This avoids a heap-use-after-free ASAN error. Close #3334

Problem     : Dead store @ 59.
Diagnostic  : Harmless issue.
Rationale   : loop is a function parameter that is not used anymore
              after this line.
Resolution  : Remove line.

Based on: http://neovim.io/doc/reports/clang/report-27475f.html#EndPath

When calling uv_spawn to launch a process set the libuv process flag
UV_PROCESS_WINDOWS_VERBATIM_ARGUMENTS to disable escaping for the
arguments otherwise libuv will wrap arguments in quotes.

- Implement `loop_schedule` method for queueing events from other threads
- Make `loop_poll_events` `recursive` static variable a field of the Loop
  structure

ref: #3188

Processing a stream's output can be queued. If stream_close() is called
before the queue is processed, the RBuffer containing the stream's data
is freed and the next read event would try to access freed memory.

To fix this behavior, use the stream's pending requests counter.

The new event processing architecture changed `jobwait()` semantics: Only one
job is processed at time since process_wait only focuses on one queue.

This fixes the problem with a few changes:

- Allow the event queue polled by `process_wait` to be overriden by a new
  argument.
- Allow the parent queue to be overriden with `queue_replace_parent`
- Create a temporary queue that serves as the parent for all jobs passed to
  `jobwait()`