kamailio-5.x: add ip translation to init

kamailio-5.x: add ip translation to init

kamailio can be started with multiple "-l" ("listen") parameters to tell
it which IPs to listen on. This can also be configured in kamailio.cfg,
of course.

This commit adds the ability to the init script to translate iface names
like "wan" into IP addresses and hand them over to kamailio as command
line arguments. This is useful when using a network connection where IPs
are dynamically assigned.

kamailio can also work with interface names, e.g. "eth0". But it may
listen to all IPs configured on the interface. To avoid this the commit
differentiates beteen IPv4 ("listen") and IPv6 ("listen6"). So if the
user wants kamailio to only listen on an IPv4 address configured on a
certain iface ("wan" for instance), he/she can just specify a list entry
"listen" with that iface.

An explanation is also added to the uci configuration file.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>