Use GetDesireableServiceFlags in static seeds, document this.

4440710 broke inserting entries into addrman from static seeds
(as well as dnsseeds which did not support service bits). Static
seeds were already being filtered by UA for 0.13.1+ (ie
NODE_WITNESS), so simply changing the default service bits to
include NODE_WITNESS (and updating docs appropriately) is
sufficient.

For DNS Seeds, we will later fix by falling back to oneshot if a
seed does not support filtering.

contrib/seeds/README.md

@@ -4,7 +4,9 @@ Utility to generate the seeds.txt list that is compiled into the client
 (see [src/chainparamsseeds.h](/src/chainparamsseeds.h) and other utilities in [contrib/seeds](/contrib/seeds)).
 
 Be sure to update `PATTERN_AGENT` in `makeseeds.py` to include the current version,
-and remove old versions as necessary.
+and remove old versions as necessary (at a minimum when GetDesireableServiceFlags
+changes its default return value, as those are the services which seeds are added
+to addrman with).
 
 The seeds compiled into the release are created from sipa's DNS seed data, like this:
 

src/net.cpp

@@ -136,7 +136,7 @@ static std::vector<CAddress> convertSeed6(const std::vector<SeedSpec6> &vSeedsIn
     for (const auto& seed_in : vSeedsIn) {
         struct in6_addr ip;
         memcpy(&ip, seed_in.addr, sizeof(ip));
-        CAddress addr(CService(ip, seed_in.port), NODE_NETWORK);
+        CAddress addr(CService(ip, seed_in.port), GetDesirableServiceFlags(NODE_NONE));
         addr.nTime = GetTime() - GetRand(nOneWeek) - nOneWeek;
         vSeedsOut.push_back(addr);
     }

src/protocol.h

@@ -291,7 +291,15 @@ enum ServiceFlags : uint64_t {
  * unless they set NODE_NETWORK_LIMITED and we are out of IBD, in which
  * case NODE_NETWORK_LIMITED suffices).
  *
- * Thus, generally, avoid calling with peerServices == NODE_NONE.
+ * Thus, generally, avoid calling with peerServices == NODE_NONE, unless
+ * state-specific flags must absolutely be avoided. When called with
+ * peerServices == NODE_NONE, the returned desirable service flags are
+ * guaranteed to not change dependant on state - ie they are suitable for
+ * use when describing peers which we know to be desirable, but for which
+ * we do not have a confirmed set of service flags.
+ *
+ * If the NODE_NONE return value is changed, contrib/seeds/makeseeds.py
+ * should be updated appropriately to filter for the same nodes.
  */
 static ServiceFlags GetDesirableServiceFlags(ServiceFlags services) {
     return ServiceFlags(NODE_NETWORK | NODE_WITNESS);