Refactor: daemons: Rearrange how regex IPC update messages work.

Before, we were simply grabbing the regex from the message, iterating
over matches, and calling attrd_send_message for each. Now, we
construct new children XML nodes for each match and later call
send_child_update on each one.

For the moment, this doesn't actually change behavior. However it's a
step towards handling regex updates from within multi-update IPC
messages.