replmd: replmd_process_link_attribute() returns type of change made

replmd: replmd_process_link_attribute() returns type of change made

In order to share work across related link attribute updates, we need
replmd_process_link_attribute() to let the caller know what actually
changed.

This patch adds an extra return type that'll be used in the next patch.
What we're interested in is: the update was ignored (i.e. it's old news),
a new link attribute was added (because this affects the overall
msg/element memory), and an existing link attribute was modified (due to
how links are actually stored, this includes deleting the link, as in
reality it simply involves setting the existing link to 'inactive').

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>