[Rpm-maint] [rpm-software-management/rpm] Add dbus-announce plugin (#1255)

[Colin Walters]

> Well, we add this on request of the DNF team that need to be notified if something else (rpm cli) changes the rpmdb underneath their daemon. 

Ah, I see.  Well, nothing else can/should be changing the rpmdb on rpm-ostree based systems (see also https://github.com/coreos/rpm-ostree/pull/1789 where rpm-ostree will start wrapping `/usr/bin/rpm`).

> So this is going to be a plugin that can be disabled or just plainly not be installed in the first place. We will probably not integrate this into the systemd-inhibit plugin but have it as a separate plugin that is very similar and comes in it's own sub package.

OK that seems fine, then we can not install it in the container image and rpm-ostree based systems.

> Can you please elaborate on why sending some DBus signals should be problematic?

A few things.  First, rpm-ostree updates are *offline* (generating a new root) by default - so anything listening and responding to these DBus signals would be confused because it's not affecting the *current* rpm database but a new one it can't see.

Second, and related to that - which exact component would be listening to these signals?  Would it be `dnf` or `libdnf`?  I really hope it's not the library because then rpm-ostree would be talking to *itself* over DBus which...yeah I would like not to have that happen :smile: 

Do we really need DBus for this versus just e.g. having interested processes use `inotify()` on the path to the rpmdb's directory, and ensuring that RPM does `touch(/path/to/rpmdb)` whenever it makes a change?  This is basically what we do for ostree changes.  (And yes in some cases rpm-ostreed does catch inotify updates from its own changes but that's kind of intentional and much less heavy-weight than talking to oneself over dbus)

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