test: make sure all handles in the loop are closed
build: fix configure when PKG_CONFIG is set

If you define the environment variable PKG_CONFIG and assign it to
pkgconf (or pkg-config), configure will say it's OK, but it won't be
used.
build: add required libraries to libuv.pc.in
windows: fall back for volume info query

Wine does not currently support FileFsVolumeInformation:

https://github.com/mirrors/wine/blob/0e42fd97c0/dlls/ntdll/file.c#L2679

So check io_status and fall back to previous behavior if not
implemented.
pipe: allow queueing pending handles

Introduce `int uv_pipe_pending_count(uv_pipe_t*)` and
`uv_handle_type uv_pipe_pending_type(uv_pipe_t*)`. They should be
used in IPC pipe's read cb to accept incoming handles:

    int count = uv_pipe_pending_count(pipe);
    int i;
    for (i = 0; i < count; i++) {
      uv_handle_type type = uv_pipe_pending_type(pipe);
      /* ... */
      uv_accept(...);
    }
test: fix compiation warning

-Wunused-function would kick in because of the close_loop function,
but there tests and benchmarks which actually don't use any loop.
windows: fix winsock status codes for address errors

- STATUS_TOO_MANY_ADDRESSES now returns WSAEADDRINUSE
- STATUS_CONFLICTING_ADDRESSES now returns WSAEADDRNOTAVAIL