MinGW 7.2 (x86_64) build failed

1. Correctly set nonblocking flag *before* calling connect(). (On glibc-Linux, we create the socket already in nonblocking mode, so this was never an issue.)
2. FDs could be leaked if connect fails. (This applies to glibc-Linux too, although connect() would have to fail immediately without blocking, which is unusual.)

These avoid the need for a read/modify/write.

This allows a lot of nice design patterns, and might later the the basis for capnp 3-party handoff within a machine.

Abstract unix socket addresses can now be specified with the
'unix-abstract:' prefix, and the 'unix:' prefix now explicitly
rejects addresses which contain NULL bytes

Since kj::StringPtr supports embedded NULs, copy its entire data
buffer to the sockaddr_un structure instead of using strcpy.
On Linux, this allows the use of the abstract socket namespace.

Re-introduce async-io.c++ containing just the default implementations of methods on the abstract interfaces.

Rename async-io.c++ to async-io-unix.c++, in preparation for factoring out common bits from async-io-win32.c++.

The implementation used by AsyncStreamFd was simply calling the general tryRead(), so why not?

Memory leak is due to compiler bug. See:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33799
http://llvm.org/bugs/show_bug.cgi?id=12286

Add missing include when building on Linux

Discovered trying to build on Android using CMake.  UIO_MAXIOV is only
exported by the linux/uio.h header which was never included.  Not sure
how normal Linux builds work.

Add missing reinterpret cast from const void* to const byte*.

Remove -pthread compile flag when building under Android.  ANDROID cmake
variable exported by android-cmake project.

Short-term fix for #220.

Fixes #221.

Fixes a build error on GNU/Hurd.

This is analagous to shutdownWrite, but for the read end of the stream

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

- Define separate times for durations vs. absolute times.
- Make more consistent with existing precedents in KJ style.