libtorc-core: Filter out our own search requests in TorcSSDP

- multicast loopback is deliberately enabled to ensure requests and
announcements reach other torc clients on the same physical device, so
filter our own searches to avoid potential net storms.
libtorc-core: Only show ip addresses for active external interface in
TorcNetwork
libtorc-core: TorcSSDP fixes and improvements

- send the correct host for ipv6 searches
- use a timer event to schedule a follow up search