Resolve hosts by DNS name in separate threads so we can cancel them

Resolve hosts by DNS name in separate threads so we can cancel them

getaddrinfo(3) may block a long time so proper termination of the daemon may
block if DNS servers are not reachable.

getaddrinfo(3) is an optional cancellation point in posix threads so it
might still block a shutdown but at least on Android (with the signal based
pthread_cancel implementation) it works, on Linux starter will kill charon
anyway after a while.