Import the latest CloudABI definitions, version 0.16.

The most important change in this release is the removal of the
poll_fd() system call; CloudABI's equivalent of kevent(). Though I think
that kqueue is a lot saner than many of its alternatives, our
experience is that emulating this system call on other systems
accurately isn't easy. It has become a complex API, even though I'm not
convinced this complexity is needed. This is why we've decided to take a
different approach, by looking one layer up.

We're currently adding an event loop to CloudABI's C library that is API
compatible with libuv (except when incompatible with Capsicum).
Initially, this event loop will be built on top of plain inefficient
poll() calls. Only after this is finished, we'll work our way backwards
and design a new set of system calls to optimize it.

Interesting challenges will include integrating asynchronous I/O into
such a system call API. libuv currently doesn't aio(4) on Linux/BSD, due
to it being unreliable and having undesired semantics.

Upgrade to CloudABI v0.17.

Compared to the previous version, v0.16, there are a couple of minor
changes:

- CLOUDABI_AT_PID: Process identifiers for CloudABI processes.

  Initially, BSD process identifiers weren't exposed inside the runtime,
  due to them being pretty much useless inside of a cluster computing
  environment. When jobs are scheduled across systems, the BSD process
  number doesn't act as an identifier. Even on individual systems they
  may recycle relatively quickly.

  With this change, the kernel will now generate a UUIDv4 when executing
  a process. These UUIDs can be obtained within the process using
  program_getpid(). Right now, FreeBSD will not attempt to store this
  value. This should of course happen at some point in time, so that it
  may be printed by administration tools.

- Removal of some unused structure members for polling.

  With the polling framework being simplified/redesigned, it turns out
  some of the structure fields were not used by the C library. We can
  remove these to keep things nice and tidy.