avahi: define missing DEFAULT_VARIANT and add conflicts

avahi: define missing DEFAULT_VARIANT and add conflicts
- Add DEFAULT_VARIANT to "avahi-daemon" and "libavahi" virtual packages;
set to "dbus" variant because it is more compatible.
When some package depends on one of these virtual packages an arbitrarily
variant it is selected. This avoids that.

- Add conflicts in order to avoid mixing packages with different variants
(when defining a DEFAULT_VARIANT the conflicts definition should be on
that side of the dependency to avoid recursive dependencies).

- Avoid building unused "dbus" dependency.

Fixes: ASU sysupgrade problem with avahi packages https://forum.openwrt.org/t/luci-attended-sysupgrade-support-thread/230552/137
Fixes: https://github.com/openwrt/packages/commit/9bc03b9d9971c2f1e146f7c2d287d1fc16e776f0
Signed-off-by: Mario Andrés Pérez <redacted>