[RFE] Support signaling only connections owned by specific uid

[Pavel Březina]

Hi dbus,

I would like to be able to connect to system bus and emit a signal. But 
I want the signal to be delivered to only processes of specific user. 
That is limit the broadcast to only names owned by the user.

So instead of sending a signal with empty destination, or unicasting it 
by providing target name, I want to be able to set the destination to 
e.g. "uid:$uid". The message bus would broadcast it only to names owned 
by $uid.

I am able to implement this change if this is something that would be 
accepted.

Rationale:
We have a daemon (sssd-kcm, Kerberos ccache manager) that runs as root 
and we would like to implement a notification mechanism (ccache changed, 
e.g. user kinited), preferably using dbus signals. However, we can not 
broadcast this information to anyone, we want to limit the signal to 
target only specific uid (the user who owns the ccache).

I understand there are different ways how to achieve this. For example, 
get the list of active names, resolve their owner and then send multiple 
unicast signals. But we would like to pursue this RFE first, if possible.

Thanks,
Pavel.