relay: Don't fail if AF_INET6 family is not supported

Someone on tor-relays@ reported that when going from 044 to 045, their config failed to work:

ORPort 587
DIRPORT 995

with :

[notice] Opening OR listener on 0.0.0.0:587
[notice] Opened OR listener connection (ready) on 0.0.0.0:587
[notice] Opening OR listener on [::]:587
[warn] Socket creation failed: Address family not supported by protocol
[notice] Opening Directory listener on 0.0.0.0:995
[notice] Closing partially-constructed OR listener connection (ready) on 
0.0.0.0:587
[notice] Closing partially-constructed Directory listener connection 
(ready) on 0.0.0.0:995

We should probably not fail this in that implicit case for IPv6 if the address is not supported.