Nest: Fix several issues with pflags

There were some confusion about validity and usage of pflags, which
caused incorrect usage after some flags from (now removed) protocol-
specific area were moved to pflags.

We state that pflags:

 - Are secondary data used by protocol-specific hooks
 - Can be changed on an existing route (in contrast to copy-on-write
   for primary data)
 - Are irrelevant for propagation (not propagated when changed)
 - Are specific to a routing table (not propagated by pipe)

The patch did these fixes:

 - Do not compare pflags in rte_same(), as they may keep cached values
   like BGP_REF_STALE, causing spurious propagation.

 - Initialize pflags to zero in rte_get_temp(), avoid initialization in
   protocol code, fixing at least two forgotten initializations (krt
   and one case in babel).

 - Improve documentation about pflags

nest/rt-dev.c

@@ -92,7 +92,6 @@ dev_ifa_notify(struct proto *P, uint flags, struct ifa *ad)
 
       a = rta_lookup(&a0);
       e = rte_get_temp(a, src);
-      e->pflags = 0;
       rte_update2(c, net, e, src);
     }
 }