Drop the config knob 'listen_address' and introduce 'listen_interface'
and 'listen_ipv6' instead.

'listen_interface' takes an openwrt interface name ('loopback', 'lan',
'wan' etc, or "*" for all), from which the primary IP is used to listen
on. If 'listen_ipv6' is set to '1', the IPv6 adress will be used, IPv4
elsewise.

procd interface triggers are now combined with this, so if the listen
interface is not yet configured when the init script is executed, the
process start is defered, and the trigger takes care of that once
the interface is ready.

Fixes #7670

Signed-off-by: Andre Heider <a.heider@gmail.com>