osx: Fix a possible segfault in uv__io_poll

In our build infrastructure, I've seen a lot of segfaults recently that
were all only happening on OSX. Upon inspecting the coredumps, it
appearded that all segfaults happened at the same instruction, and upon
translating the assembly back to the source, I found that an array could
be indexed with a -1 index before the index was checked to be not -1.

As concrete evidence, here is the situation that I found caused the
segfault.  The instruction in question along with the relevant register
values was:

    mov    (%r8,%r15,8),%r12

    r8  = 0x7fb0ba800000
    r15 = 0xffffffffffffffff

    r8 + r15 * 8 == 0x7fb0ba7ffff8

It appears that the base of loop->watchers was page aligned, and by
going back one word I guess that the page wasn't mapped, causing our
segfaults.
pipe: allow queueing pending handles

If multiple handles arrive to the IPC pipe at the same time (happens on
some platforms), libuv will queue them internally, and call `read2_cb`
multiple times with a null-buffer and proper `handle_type`.
Revert "pipe: allow queueing pending handles"

The commit is quite broken and must be refactored before going into.

This reverts commit 08aeaf61935060e01f15cc1df2c9f09e892b0980.
tcp: uv_tcp_dualstack()

Explicitly disable/enable dualstack depending on presence of flag set by
uv_tcp_dualstack() function.