MFC r321514, r322885, r323015, r323177

This brings the CloudABI code more or less in sync with HEAD.

r321514:
Upgrade to the latest sources generated from the CloudABI specification.

The CloudABI specification has had some minor changes over the last half
year. No substantial features have been added, but some features that
are deemed unnecessary in retrospect have been removed:

• mlock()/munlock():
  
  These calls tend to be used for two different purposes: real-time support and handling of sensitive (cryptographic) material that shouldn't end up in swap. The former use case is out of scope for CloudABI. The latter may also be handled by encrypting swap.
  
  Removing this has the advantage that we no longer need to worry about having resource limits put in place.

• SOCK_SEQPACKET:
  
  Support for SOCK_SEQPACKET is rather inconsistent across various operating systems. Some operating systems supported by CloudABI (e.g., macOS) don't support it at all. Considering that they are rarely used, remove support for the time being.

• getsockname(), getpeername(), etc.:
  
  A shortcoming of the sockets API is that it doesn't allow you to create socket(pair)s, having fake socket addresses associated with them. This makes it harder to test applications or transparently forward (proxy) connections to them.
  
  With CloudABI, we're slowly moving networking connectivity into a separate daemon called Flower. In addition to passing around socket file descriptors, this daemon provides address information in the form of arbitrary string labels. There is thus no longer any need for requesting socket address information from the kernel itself.

This change also updates consumers of the generated code accordingly.
Even though system calls end up getting renumbered, this won't cause any
problems in practice. CloudABI programs always call into the kernel
through a kernel-supplied vDSO that has the numbers updated as well.

Obtained from: https://github.com/NuxiNL/cloudabi

r322885:
Sync CloudABI compatibility against the latest upstream version (v0.13).

With Flower (CloudABI's network connection daemon) becoming more
complete, there is no longer any need for creating any unconnected
sockets. Socket pairs in combination with file descriptor passing is all
that is necessary, as that is what is used by Flower to pass network
connections from the public internet to listening processes.

Remove all of the kernel bits that were used to implement socket(),
listen(), bindat() and connectat(). In principle, accept() and
SO_ACCEPTCONN may also be removed, but there are still some consumers
left.

Obtained from: https://github.com/NuxiNL/cloudabi

r323015:
Complete the CloudABI networking refactoring.

Now that all of the packaged software has been adjusted to either use
Flower (https://github.com/NuxiNL/flower) for making incoming/outgoing
network connections or can have connections injected, there is no longer
need to keep accept() around. It is now a lot easier to write networked
services that are address family independent, dual-stack, testable, etc.

Remove all of the bits related to accept(), but also to
getsockopt(SO_ACCEPTCONN).

r323177:
Merge pipes and socket pairs.

Now that CloudABI's sockets API has been changed to be addressless and
only connected socket instances are used (e.g., socket pairs), they have
become fairly similar to pipes. The only differences on CloudABI is that
socket pairs additionally support shutdown(), send() and recv().

To simplify the ABI, we've therefore decided to remove pipes as a
separate file descriptor type and just let pipe() return a socket pair
of type SOCK_STREAM. S_ISFIFO() and S_ISSOCK() are now defined
identically.