config-parser: Clarify how <allow>, <deny> attributes work

config-parser: Clarify how <allow>, <deny> attributes work

The giant conditionals used to check policy attributes are increasingly
unwieldy, so let's try something else. Bundle together the send_
attributes, the receive_ attributes, the eavesdrop attribute
(which can go on either send or receive rules) and the other attributes
into equivalence classes, and write the conditionals in terms of those
equivalence classes.

In particular, this correctly forbids
    <allow receive_type="..." send_destination="..."/>
which was previously allowed but nonsensical (the send part took
precedence and the receive part was ignored).

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: Thiago Macieira <thiago@kde.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=92853