af_unix allows for sockets to be bound to a name that is autogenerated.
Currently this type of binding is only supported by a very generic
rule.

  unix (bind) type=dgram,

but this allows both sockets with specified names and anonymous
sockets. Extend unix rule syntax to support specifying just an
auto bind socket by specifying addr=auto

eg.

  unix (bind) addr=auto,

It is important to note that addr=auto only works for the bind
permission as once the socket is bound to an autogenerated address,
the addr with have a valid unique value that can be matched against
with a regular

  addr=@name

expression

Fixes: https://bugs.launchpad.net/apparmor/+bug/1867216
MR: !521


Signed-off-by: John Johansen <john.johansen@canonical.com>