Notes feature

[jminor]

It would be great if there was a per-basename, per-host notes field, where administrators could leave a short text note for other admins. This comes up, for example, when someone has frozen a basename on a host. Often another admin will wonder, "Why was this frozen 3 weeks ago? Should I unfreeze it?" Allowing a notes field would let you explain why you froze something.

Proposed usage:
% d3 -n <message> <basename>
Sets the note message for the given <basename> on this host.
If <message> is the empty string, the note is cleared.

% d3 -n <message> [other actons]
Sets the note message for the given basename on this host in addition to doing [other actions]

For example:
% d3 -n "no upgrades until Sally returns from her overseas trip" freeze some-app

The list-* actions should display the note (if any) for each basename.