evdns: New flag to make evdns not prevent the event loop from exiting

Here is the brief description of problem:
When you are use evdns to resolve domains to IP adresses (see
./sample/dns-example) you loop never returns from event_base_dispatch(),
and because of this the program will never terminated.

Because existing programs may be depending on the old behavior, we
only apply the fix when evdns_base_new() is created with a new flag -
EVDNS_BASE_DISABLE_WHEN_INACTIVE.

 (Commit message edited by Nick while squashing the branch.)
Merge branch '21_evdns_disable_when_inactive'