Fix problem with the sockaddr struct.

Otherwise, at least clang++, forward-declares it inside the namespace `::capnp` and doesn't match it with a real `struct sockaddr` from the header 'socket.h'.

Also, accept `const` pointer - there is no point not to do so.

capnpc-capnp: keep list of extended interfaces.

Fix two typos in bit operations.

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

fix some error messages

Apparently writev() imposes a limit on the vector size.  Work around that.  Not sure why libc doesn't already do this.

Make it possible for a message builder to incorporate a byte array not owned by it without copying.  Useful for injecting large data blobs into a message without copying their contents.  Access this functionality through the Orphanage interface.

Extend FlatArrayMessageReader to indicate where the message actually ended, in case it was not the end of the array.  Useful when reading an mmap'd file with a Cap'n Proto prefix.  Also fix a bug in Maybe dealing with DisallowConstCopy subclasses.

Cleaner disconnect handling.  Better fix for issue #71, and also simplifies the interface and improves robustness.