Refactor: daemons: Recursively handle regex IPC update messages.

This just uses attrd_client_update to handle the big expanded regex IPC
message, instead of calling send_child_update for each part. This moves
us closer to handling regexes the same way other update messages are
handled.

This should also allow regexes to be handled atomically (assuming the
attribute daemon supports that), and for local sync points to work with
regexes.

It is difficult to say whether cluster-wide sync points also work now.
The log files make it look like they do, but that could just be due to
my simple cluster setup. It looks like cluster-wide sync points for
regexes will work as long as they also work for multi-update IPC
messages (like the previous atomic update work). It also looks like
cluster-wide sync points will not work if the cluster doesn't support
multi-update IPC messages (see T601).

With that in mind, I think I'm willing to declare that T600 is fixed.

Fixes T600