dnsmasq: add a new "bind" configuration option

dnsmasq: add a new "bind" configuration option

The nonwildcard option only allows to choose between the default behaviour (bind
to the wildcard address only, which unfeasible when running multiple instances
of dnsmasq) and --bind-dynamic (bind to any existing/new interfaces which aren't
explicitly configured otherwise). Add a new "bind" option, which can take the
value of "dynamic" or "interfaces", allowing the user to additionally choose the
--bind-interfaces behaviour (bind only to explicitly configured interfaces).

The dnsmasq man page [1] contains more detailed information about these
arguments. Finally, the nonwildcard option is kept for backwards compatibility.

[1] https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>