sbus: Add sbus_request context

struct sbus_request represents a request from a dbus client
being handled by a dbus server implementation. The struct
contains the message, connection and method which is being
invoked.

In the future it will contain caller information as well.

sbus_request is a talloc memory context, and is a good place to
attach any allocations and memory specific to the request.

Each handler accepts an sbus_request. If a handler returns
EOK, it is assumed that the handler will finish the request.
Any of the sbus_request_*finish() methods can be used to
complete the request and send back a reply.

This patch migrates the monitor and data provider dbus code
to use sbus_request, but does not try to rework the talloc
context's to use it.
sbus: Refactor how we export DBus interfaces

Most importantly, stop using per connection private data. This doesn't
scale when you have more than one thing exporting or exported on a
connection.

Remove struct sbus_interface and expand sbus_conn_add_interface()
function. Remove various struct sbus_interface args to connection
initialization functions and make callers use sbus_conn_add_interface()
directly. The old method was optimized for exporting one interface
on a connection. We'll have connections that export zero, one or more
interfaces

To export an interface on a DBus server, call sbus_conn_add_interface()
from within the sbus_server_conn_init_fn. To export an interface on
a DBus client, call sbus_conn_add_interface() after sbus_new_connection()
returns.

However, we now have type safe interface exporting in the various
high level sss_process_init() sss_monitor_init() and so on.

Introspection support was not in use, and is now gone until we
implement it using the metadata (future patch).
sbus: Make sbus_new_server() work for non-priveleged processes

I'd like to use this during testing. We should check that the
socket ownership is the same as the process, rather than limiting
it to root.
sbus_codegen_tests: Add some testing of dispatch and handler code

This starts a DBus server with some handlers, and runs some method
calls against it.