p2p: Don't query DNS seeds when both IPv4 and IPv6 are unreachable

This happens, for example, if the user specified -onlynet=onion or
-onlynet=i2p. DNS seeds only resolve to IPv4 / IPv6 addresses,
making their answers useless to us, since we don't want to make
connections to these.
If, within the DNS seed thread, we'd instead do fallback AddrFetch
connections to one of the clearnet addresses the DNS seed resolves to,
we might get usable addresses from other networks
if lucky, but would be violating our -onlynet user preference
in doing so.

Therefore, in this case it is better to rely on fixed seeds for networks we
want to connect to.

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

test/functional/feature_config_args.py

@@ -186,11 +186,12 @@ class ConfArgsTest(BitcoinTestFramework):
         with self.nodes[0].assert_debug_log(expected_msgs=[
                 "Loaded 0 addresses from peers.dat",
                 "DNS seeding disabled",
-                "Adding fixed seeds as -dnsseed=0, -addnode is not provided and all -seednode(s) attempted\n",
+                "Adding fixed seeds as -dnsseed=0 (or IPv4/IPv6 connections are disabled via -onlynet), -addnode is not provided and all -seednode(s) attempted\n",
         ]):
             self.start_node(0, extra_args=['-dnsseed=0', '-fixedseeds=1'])
         assert time.time() - start < 60
         self.stop_node(0)
+        self.nodes[0].assert_start_raises_init_error(['-dnsseed=1', '-onlynet=i2p', '-i2psam=127.0.0.1:7656'], "Error: Incompatible options: -dnsseed=1 was explicitly specified, but -onlynet forbids connections to IPv4/IPv6")
 
         # No peers.dat exists and dns seeds are disabled.
         # We expect the node will not add fixed seeds when explicitly disabled.