Add documentation on Linux ipc abstract namespace support and a bug fix.

I documented the abstract namespace support, as requested.  See issue [567](https://zeromq.jira.com/browse/LIBZMQ-567 "libzmq issue #567").

I also realized that the abstract name detection needed to look at the first two characters, not only the first.  And I fixed the address length calculation after seeing how it looked with netstat and in /proc/net/unix.  The address was including all the trailing NULL characters so that the pathname was always 108 characters including all the NULL bytes.  The address length calculation now trims the length down to include only the NULL-terminated pathname.  Not only does this look nicer, it is the expected behavior.