dnsmasq: distinct Ubus names for multiple instances

dnsmasq: distinct Ubus names for multiple instances

Currently, when using multiple dnsmasq instances they are all assigned
to the same Ubus instance name. This does not work, as only a single
instance can register with Ubus at a time. In the log, this leads to
`Cannot add object to UBus: Invalid argument` error messages.
Furthermore, upstream 3c93e8eb41952a9c91699386132d6fe83050e9be changes
behaviour so that instead of the log, dnsmasq exits at start instead.

With this patch, all dnsmasq instances are assigned unique names so that
they can register with Ubus concurrently. One of the enabled instances
is always assigned the previous default name "dnsmasq" to avoid breaking
backwards compatibility with other software relying on that default.
Previously, a random instance got assigned that name (while the others
produced error logs). Now, the first unnamed dnsmasq config section is
assigned the default name. If there are no unnamed dnsmasq sections the
first encountered named dnsmasq config section is assigned instead.

A similar issue exists for Dbus and was similarly addressed.

Signed-off-by: Etan Kissling <etan.kissling@gmail.com>
[tweaked commit message] dnsmasq was not crashing it is exiting
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
(cherry picked from commit ba5bd8e556b2e7573d27b16e005ba287e066f795)